bpo-36374: Fix a possible null pointer dereference (GH-12449)

https://bugs.python.org/issue36374

Misc/NEWS.d/next/Core and Builtins/2019-03-19-15-46-42.bpo-36374.EWKMZE.rst

+Fix a possible null pointer dereference in ``merge_consts_recursive()``.
+Patch by Zackery Spytz.

Python/compile.c

@@ -1210,7 +1210,7 @@ merge_consts_recursive(struct compiler *c, PyObject *o)
     PyObject *t = PyDict_SetDefault(c->c_const_cache, key, key);
     if (t != key) {
         // o is registered in c_const_cache.  Just use it.
-        Py_INCREF(t);
+        Py_XINCREF(t);
         Py_DECREF(key);
         return t;
     }