Coverity: Fix the forward_null error in Python binding codes

Related coverity log:

Error: FORWARD_NULL:
/builddir/build/BUILD/libvirt-0.9.10/python/libvirt-override.c:355:
assign_zero: Assigning: "params" = 0.
/builddir/build/BUILD/libvirt-0.9.10/python/libvirt-override.c:458:
var_deref_model: Passing null variable "params" to function
"getPyVirTypedParameter", which dereferences it. (The dereference is assumed on
the basis of the 'nonnull' parameter attribute.)

python/libvirt-override.c

@@ -71,7 +71,7 @@ static char *py_str(PyObject *obj)
 /* Helper function to convert a virTypedParameter output array into a
  * Python dictionary for return to the user.  Return NULL on failure,
  * after raising a python exception.  */
-static PyObject * ATTRIBUTE_NONNULL(1)
+static PyObject *
 getPyVirTypedParameter(const virTypedParameterPtr params, int nparams)
 {
     PyObject *key, *val, *info;