Fix undefined behaviour in datetime.time.fromisoformat() (#111982)

Fix undefined behaviour in datetime.time.fromisoformat() (#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).