gallium/u_queue: fix a crash with atexit handlers

gallium/u_queue: fix a crash with atexit handlers

Commit 4aea8fe ("gallium/u_queue: fix random crashes when the app calls
exit()") added a atexit handler which calls
util_queue_killall_and_wait() for each queue to stop the threads.
However the app is also free to use atexit handlers to clean up things,
leading to util_queue_destroy() call which will also call
util_queue_killall_and_wait() for the same queue again, causing threads
being joined twice, and that is undefined. This happens with libglut,
for example. A simple fix is to just set num_threads to 0 as there are
no more valid threads after util_queue_killall_and_wait() returns.

Fixes: 4aea8fe "gallium/u_queue: fix random crashes when the app calls exit()"
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>