CPython shouldn't specialize function calls with custom _PyFunction_Vectorcall

Summary:
CPython includes a check in the interpreter loop to not go through vectorcall if it's been customized. But it'll specialize these method calls, and then avoid the custom hook.

It should refuse to specialize if the hook has been set.

Reviewed By: alexmalyshev

Differential Revision: D67505322

fbshipit-source-id: 8bc9c26b961e168113d0d23a6a40e653f4f0997c

Author: DinoV

Python/specialize.c

@@ -1678,6 +1678,9 @@ specialize_py_call(PyFunctionObject *func, _Py_CODEUNIT *instr, int nargs,
         SPECIALIZATION_FAIL(CALL, SPEC_FAIL_CALL_PEP_523);
         return -1;
     }
+    if (func->vectorcall != _PyFunction_Vectorcall) {
+        return -1;
+    }
     if (kwnames) {
         SPECIALIZATION_FAIL(CALL, SPEC_FAIL_CALL_KWNAMES);
         return -1;