[3.15] Fix a dangling pointer and reference leak in local_timezone_from_timestamp (GH-156598) (GH-156643)

Fix a dangling pointer and reference leak in `local_timezone_from_timestamp` (GH-156598)
(cherry picked from commit fcf0a98)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Modules/_datetimemodule.c

@@ -6767,6 +6767,9 @@ local_timezone_from_timestamp(time_t timestamp)
     struct tm local_time_tm;
     PyObject *nameo = NULL;
     const char *zone = NULL;
+#ifndef HAVE_STRUCT_TM_TM_ZONE
+    char buf[100];  // for zone, which is used after the block below
+#endif
 
     if (_PyTime_localtime(timestamp, &local_time_tm) != 0)
         return NULL;
@@ -6777,9 +6780,9 @@ local_timezone_from_timestamp(time_t timestamp)
     {
         PyObject *local_time, *utc_time;
         struct tm utc_time_tm;
-        char buf[100];
-        strftime(buf, sizeof(buf), "%Z", &local_time_tm);
-        zone = buf;
+        if (strftime(buf, sizeof(buf), "%Z", &local_time_tm) != 0) {
+            zone = buf;
+        }
         local_time = new_datetime(local_time_tm.tm_year + 1900,
                                   local_time_tm.tm_mon + 1,
                                   local_time_tm.tm_mday,
@@ -6789,8 +6792,10 @@ local_timezone_from_timestamp(time_t timestamp)
         if (local_time == NULL) {
             return NULL;
         }
-        if (_PyTime_gmtime(timestamp, &utc_time_tm) != 0)
+        if (_PyTime_gmtime(timestamp, &utc_time_tm) != 0) {
+            Py_DECREF(local_time);
             return NULL;
+        }
         utc_time = new_datetime(utc_time_tm.tm_year + 1900,
                                 utc_time_tm.tm_mon + 1,
                                 utc_time_tm.tm_mday,