[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 21615f77b5a580e83589abae618dbe7c298700e2)

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