configure: Explicitly disable w32threads if the test for it fails

This avoids false positive enabling of w32threads if explicitly
requested on the command line, but dependencies are missing.
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  configure: Explicitly disable w32threads if the test for it fails

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/shorten: clear bitstream buffer

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f3ca95606fb_6393_luckynight-partial.shn
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>