[PATCH] msvcrt: Don't crash if _vsnwprintf gets NULL as format string

[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=33350

Your paranoid android.


=== wxppro (32 bit printf) ===
09e8:printf: unhandled exception c0000005 at 77C431F7

=== w2003std (32 bit printf) ===
05f0:printf: unhandled exception c0000005 at 77BD4190