bpo-36421: Fix a possible double decref in _ctypes.c's PyCArrayType_new(). (GH-12530)

Set type_attr to NULL after the assignment to stgdict->proto (like
what is done with stgdict after the Py_SETREF() call) so that it is
not decrefed twice on error.

Misc/NEWS.d/next/Core and Builtins/2019-03-24-21-33-22.bpo-36421.gJ2Pv9.rst

+Fix a possible double decref in _ctypes.c's ``PyCArrayType_new()``.

Modules/_ctypes/_ctypes.c

@@ -1522,6 +1522,7 @@ PyCArrayType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     stgdict->align = itemalign;
     stgdict->length = length;
     stgdict->proto = type_attr;
+    type_attr = NULL;
 
     stgdict->paramfunc = &PyCArrayType_paramfunc;