bpo-31347: _PyObject_FastCall_Prepend: do not call memcpy if args might not be null (#3329)

Passing NULL as the second argument to to memcpy is undefined behavior even if the size is 0.

Misc/NEWS.d/next/Core and Builtins/2017-09-04-16-35-06.bpo-31347.KDuf2w.rst

+Fix possible undefined behavior in _PyObject_FastCall_Prepend.

Objects/call.c

@@ -854,9 +854,9 @@ _PyObject_FastCall_Prepend(PyObject *callable,
 
     /* use borrowed references */
     args2[0] = obj;
-    memcpy(&args2[1],
-           args,
-           (nargs - 1)* sizeof(PyObject *));
+    if (nargs > 1) {
+        memcpy(&args2[1], args, (nargs - 1) * sizeof(PyObject *));
+    }
 
     result = _PyObject_FastCall(callable, args2, nargs);
     if (args2 != small_stack) {