merge 3.4 (#25245)
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index c3f1896..123f14b 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -1193,6 +1193,7 @@
 {
     Py_ssize_t num_read;
 
+    *s = NULL;
     if (self->next_read_idx > PY_SSIZE_T_MAX - n) {
         PickleState *st = _Pickle_GetGlobalState();
         PyErr_SetString(st->UnpicklingError,