[PATCH] schannel/tests: Avoid dereferencing a null function pointer

[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=35342

Your paranoid android.


=== w8 (32 bit main) ===
main.c:222: Test failed: status: 0x74be157d

=== w864 (32 bit main) ===
main.c:222: Test failed: status: 0x745b157d

=== w1064 (32 bit main) ===
main.c:222: Test failed: status: 0x701cadf0

=== w864 (64 bit main) ===
main.c:222: Test failed: status: 0x22091010

=== w1064 (64 bit main) ===
main.c:222: Test failed: status: 0x39a4e600