Fix "use after free" issue in `essential_solver.cpp`

The address sanitizer highlighted this issue in our code base. It
looks like the code is currently grabbing a pointer to a temporary
object and then performing operations on it.

I printed some information right before the asan crash:

    eigensolver address: 0x7f0ad95032f0
    eigensolver size: 4528
    eig_vecs_ ptr: 0x7f0ad95045e0
    eig_vecs_ offset: 4848

This shows that `eig_vecs_` points past the end of `eigensolver`. In
other words, it points at the temporary object created by the
`eigensolver.eigenvectors()` call.

Compare the docs for `.eigenvalues()`:
https://eigen.tuxfamily.org/dox/classEigen_1_1EigenSolver.html#a0f507ad7ab14797882f474ca8f2773e7
to the docs for `.eigenvectors()`:
https://eigen.tuxfamily.org/dox/classEigen_1_1EigenSolver.html#a66288022802172e3ee059283b26201d7

The difference in return types is interesting. `.eigenvalues()`
returns a reference. But `.eigenvectors()` returns a matrix.

This patch here fixes the problem by saving the temporary object and
then grabbing a pointer into it.

This is a curated snippet of the original asan failure:

    ==12==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7fc633704640 at pc 0x7fc64f7f1593 bp 0x7ffe8875fc90 sp 0x7ffe8875fc88
    READ of size 8 at 0x7fc633704640 thread T0
        #0 0x7fc64f7f1592 in cv::usac::EssentialMinimalSolverStewenius5ptsImpl::estimate(std::__1::vector<int, std::__1::allocator<int> > const&, std::__1::vector<cv::Mat, std::__1::allocator<cv::Mat> >&) const /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/usac/essential_solver.cpp:181:48
        #1 0x7fc64f915d92 in cv::usac::EssentialEstimatorImpl::estimateModels(std::__1::vector<int, std::__1::allocator<int> > const&, std::__1::vector<cv::Mat, std::__1::allocator<cv::Mat> >&) const /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/usac/estimator.cpp:110:46
        #2 0x7fc64fa74fb0 in cv::usac::Ransac::run(cv::Ptr<cv::usac::RansacOutput>&) /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/usac/ransac_solvers.cpp:152:58
        #3 0x7fc64fa6cd8e in cv::usac::run(cv::Ptr<cv::usac::Model const> const&, cv::_InputArray const&, cv::_InputArray const&, int, cv::Ptr<cv::usac::RansacOutput>&, cv::_InputArray const&, cv::_InputArray const&, cv::_InputArray const&, cv::_InputArray const&) /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/usac/ransac_solvers.cpp:1010:16
        #4 0x7fc64fa6fb46 in cv::usac::findEssentialMat(cv::_InputArray const&, cv::_InputArray const&, cv::_InputArray const&, int, double, double, cv::_OutputArray const&) /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/usac/ransac_solvers.cpp:527:9
        #5 0x7fc64f3b5522 in cv::findEssentialMat(cv::_InputArray const&, cv::_InputArray const&, cv::_InputArray const&, int, double, double, int, cv::_OutputArray const&) /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/five-point.cpp:437:16
        #6 0x7fc64f3b7e00 in cv::findEssentialMat(cv::_InputArray const&, cv::_InputArray const&, cv::_InputArray const&, int, double, double, cv::_OutputArray const&) /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/five-point.cpp:486:12
        ...

    Address 0x7fc633704640 is located in stack of thread T0 at offset 17984 in frame
        #0 0x7fc64f7ed4ff in cv::usac::EssentialMinimalSolverStewenius5ptsImpl::estimate(std::__1::vector<int, std::__1::allocator<int> > const&, std::__1::vector<cv::Mat, std::__1::allocator<cv::Mat> >&) const /proc/self/cwd/external/com_github_opencv_opencv/modules/calib3d/src/usac/essential_solver.cpp:36

      This frame has 63 object(s):
        [32, 56) 'coefficients' (line 38)
        [96, 384) 'ee' (line 55)
        ...
        [13040, 17568) 'eigensolver' (line 142)
        [17824, 17840) 'ref.tmp518' (line 143)
        [17856, 17872) 'ref.tmp523' (line 144)
        [17888, 19488) 'ref.tmp524' (line 144) <== Memory access at offset 17984 is inside this variable
        [19616, 19640) 'ref.tmp532' (line 169)
        ...

The crash report says that we're accessing a temporary object from
line 144 when we shouldn't be. Line 144 looks like this:
https://github.com/opencv/opencv/blob/4.6.0/modules/calib3d/src/usac/essential_solver.cpp#L144

    const auto * const eig_vecs_ = (double *) eigensolver.eigenvectors().real().data();

We are using version 4.6.0 for this, but the problem is present on the
4.x branch.

Note that I am dropping the .real() call here. I think that is safe because
of the code further down (line 277 in the most recent version):

    const int eig_i = 20 * i + 12; // eigen stores imaginary values too

The code appears to expect to have to skip doubles for the imaginary parts
of the complex numbers.

Admittedly, I couldn't find a test case that exercised this code path to
validate correctness.

modules/calib3d/src/usac/essential_solver.cpp

@@ -239,7 +239,8 @@ public:
             // (5) Compute the left eigenvectors of the action matrix
             Eigen::EigenSolver<Eigen::Matrix<double, 10, 10>> eigensolver(action_mat_eig);
             const Eigen::VectorXcd &eigenvalues = eigensolver.eigenvalues();
-            const auto * const eig_vecs_ = (double *) eigensolver.eigenvectors().real().data();
+            const Eigen::MatrixXcd eigenvectors = eigensolver.eigenvectors();
+            const auto * const eig_vecs_ = (double *) eigenvectors.data();
 #else
             Matx<double, 10, 10> A = constraint_mat.colRange(0, 10),
                              B = constraint_mat.colRange(10, 20), eliminated_mat;