Clarify return value of PyLong_AsLongLong().

The function is documented to return -1 on error.  If res was < 0, it
returned res.  It wasn't clear that the invariant was res < 0 iff res
== -1.

Objects/longobject.c

@@ -696,7 +696,7 @@ PyLong_AsLongLong(PyObject *vv)
 
 	/* Plan 9 can't handle LONG_LONG in ? : expressions */
 	if (res < 0)
-		return (LONG_LONG)res;
+		return (LONG_LONG)-1;
 	else
 		return bytes;
 }