Oops. Return -1 to distinguish error from empty dict.

This change probably isn't work a bug fix.  It's unlikely that anyone
was calling this method without passing it a real dict.

Objects/dictobject.c

@@ -1256,7 +1256,7 @@ PyDict_Size(PyObject *mp)
 {
 	if (mp == NULL || !PyDict_Check(mp)) {
 		PyErr_BadInternalCall();
-		return 0;
+		return -1;
 	}
 	return ((dictobject *)mp)->ma_used;
 }