Fix two compiler warnings in Win32 sound plugin support.
Fix two compiler warnings in Win32 sound plugin support.

The first one is benign:

../../platforms/win32/plugins/SoundPlugin/sqWin32Sound.c:76:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^

The second one is more suspicious: we assign twice the variable in the same instruction, which is problematic...
We could have replaced by playBufferIndex = (playBufferIndex+1) & 1;
But we use the xor instead as we now that the variable is already 0 or 1.

../../platforms/win32/plugins/SoundPlugin/sqWin32Sound.c:116:25: warning: multiple unsequenced modifications to 'playBufferIndex' [-Wunsequenced]
      playBufferIndex = ++playBufferIndex & 1;
                      ~ ^
../../platforms/win32/plugins/SoundPlugin/sqWin32Sound.c:127:24: warning: multiple unsequenced modifications to 'recBufferIndex' [-Wunsequenced]
      recBufferIndex = ++recBufferIndex & 1;
                     ~ ^

timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,bencoman/pharo-vm,peteruhnak/pharo-vm,bencoman/pharo-vm,timfel/squeakvm,bencoman/pharo-vm,peteruhnak/pharo-vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,OpenSmalltalk/vm,peteruhnak/pharo-vm,timfel/squeakvm,timfel/squeakvm,peteruhnak/pharo-vm,bencoman/pharo-vm,bencoman/pharo-vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,peteruhnak/pharo-vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm,bencoman/pharo-vm,bencoman/pharo-vm,OpenSmalltalk/vm