on signed integer overflow wrapping. Otherwise mktime(3) and timegm(3)
can hang, in case the timestamp passed in struct tm is not representable
in a time_t. Specifically, any timestamp after 2038-01-19 03:14:07, in
combination with a 32-bit time_t.
Note that it would be better to change the code to not rely on undefined
behaviour, but it is contributed code, and it is not entirely trivial to
fix the issue properly.
MFC after: 3 days
7e4de3c228