Don't try to destroy NULL condition variables

This avoids a segfault during error-unwinding if an invalid display name
is passed to XOpenDisplay().

Fixes: 8a368d80 "Fix two memory leaks in _XFreeX11XCBStructure()"
Resolves: #155
Signed-off-by: Simon McVittie <smcv@debian.org>
6 jobs for issue155 in 6 minutes and 4 seconds (queued for 1 second)