python: plug memory leak on libvirt_virConnectOpenAuth

* Detected by valgrind. Leak introduced in commit 5ab109f8.

* python/libvirt-override.c: avoid memory leak on libvirt_virConnectOpenAuth.

* How to reproduce?

  % valgrind -v --leak-check=full virt-clone --print-xml
  Note: it can hit the issue although options are incomplete.

* Actual valgrind result:

==1801== 12 bytes in 1 blocks are definitely lost in loss record 25 of 3,270
==1801==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==1801==    by 0xCF1F60E: libvirt_virConnectOpenAuth (libvirt-override.c:1507)
==1801==    by 0x3AFEEDE7F3: PyEval_EvalFrameEx (ceval.c:3794)
==1801==    by 0x3AFEEDF99E: PyEval_EvalFrameEx (ceval.c:3880)
==1801==    by 0x3AFEEDF99E: PyEval_EvalFrameEx (ceval.c:3880)
==1801==    by 0x3AFEEDF99E: PyEval_EvalFrameEx (ceval.c:3880)
==1801==    by 0x3AFEEDF99E: PyEval_EvalFrameEx (ceval.c:3880)
==1801==    by 0x3AFEEE0466: PyEval_EvalCodeEx (ceval.c:3044)
==1801==    by 0x3AFEEE0541: PyEval_EvalCode (ceval.c:545)
==1801==    by 0x3AFEEFB88B: run_mod (pythonrun.c:1351)
==1801==    by 0x3AFEEFB95F: PyRun_FileExFlags (pythonrun.c:1337)
==1801==    by 0x3AFEEFCE4B: PyRun_SimpleFileExFlags (pythonrun.c:941)
Signed-off-by: NAlex Jia <ajia@redhat.com>

python/libvirt-override.c

@@ -1520,6 +1520,7 @@ libvirt_virConnectOpenAuth(PyObject *self ATTRIBUTE_UNUSED, PyObject *args) {
 
     c_retval = virConnectOpenAuth(name, &auth, flags);
     LIBVIRT_END_ALLOW_THREADS;
+    free(auth.credtype);
     py_retval = libvirt_virConnectPtrWrap((virConnectPtr) c_retval);
     return(py_retval);
 }