For VCL_TIME values that would convert to a year element that can not fit
in an int, gmtime_r would fail, and VTIM_format() would use random stack
values when picking weekday and month strings.

This patch changes VTIM_format to return "" when gmtime_r reports
failures. This way the API is not changed. Callers can test for empty
string to catch the failure if needed.

VRT_TIME_string is patched to catch the VTIM_format error, and return NULL
on failure.

Fixes: #3308