[tz] Invalid tm handling in %s pattern in strftime

Hi Paul, https://github.com/eggert/tz/commit/8b238ec54c09556eb2aa405c1741eedfd12c4a87... added validation of tm argument. I wonder why localtime_r call is done: 1) According to the man page mktime sets errno if tm is invalid. Why is it not safe to just check errno value (saving and restoring it if needed)? 2) man page says that in error case mktime "returns (time_t) -1 and does not alter the members of the broken-down time structure". Wouldn't it be enough to check that tm.tm_yday is >= 0? Thanks, Almaz