[3.12] Fix undefined behaviour in datetime.time.fromisoformat() (GH-111982) (#111992)

Fix undefined behaviour in datetime.time.fromisoformat() (GH-111982)

Fix undefined behaviour in datetime.time.fromisoformat() when parsing a string without a timezone. 'tzoffset' is not assigned to by parse_isoformat_time if it returns 0, but time_fromisoformat then passes tzoffset to another function, which is undefined behaviour (even if the function in question does not use the value).
(cherry picked from commit 21615f77b5)

Co-authored-by: T. Wouters <thomas@python.org>

Modules/_datetimemodule.c

@@ -4620,7 +4620,7 @@ time_fromisoformat(PyObject *cls, PyObject *tstr) {
     }
 
     int hour = 0, minute = 0, second = 0, microsecond = 0;
-    int tzoffset, tzimicrosecond = 0;
+    int tzoffset = 0, tzimicrosecond = 0;
     int rv = parse_isoformat_time(p, len,
                                   &hour, &minute, &second, &microsecond,
                                   &tzoffset, &tzimicrosecond);