Fix display locking on one thread...
Fix display locking on one thread...

... caused by incorrect check.

Also we don't actually need to exit and join a thread. Doing it usually
causes glibc to leak (unix), and since we control it other way around
there is not point to do this, especially when we don't really check
function return.

MrKrzYch00/zopfli,MrKrzYch00/zopfli