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. (cherry picked from commit 5e33378) Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Author: miss-islington

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

@@ -0,0 +1 @@
+Fix a possible double decref in _ctypes.c's ``PyCArrayType_new()``.

Modules/_ctypes/_ctypes.c

@@ -1481,6 +1481,7 @@ 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;