fix a crash in debug builds where the compiler did not optimize

fix a crash in debug builds where the compiler did not optimize
ch->sfx-> to use a local variable holding the unchanging value of sfx
(the crash occurs when the mixer thread clears ch->sfx while the main
thread is deciding whether to merge a sound onto an existing channel -
if the channel dies during this code, the sfx pointer must be preserved)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11161 d7cf8633-e32d-0410-b094-e92efae38249