msvcrt: Don't crash on NULL struct _tm in asctime

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=6983

Your paranoid android.


=== w2000pro (32 bit time) ===
time: unhandled exception c0000005 at 78028AD5

=== wxppro (32 bit time) ===
time: unhandled exception c0000005 at 77C4A8B6