gpu-setup: Tolerate empty pid files and pid file removal

Because job-init is asynchronous to mxqd (see previous commit),
job-release rases with it, too.

Tolerate empty pid files as well as a pid files being removed away from
under us.

helper/gpu-setup

@@ -210,7 +210,7 @@ job_release() {
     test -d /dev/shm/mxqd/gpu_devs || die "$0: Not initialized (no dir /dev/shm/mxqd/gpu_devs)"
     for d in /dev/shm/mxqd/gpu_devs/???; do
         if [ -e $d/pid ]; then
-            test_pid=$(cat $d/pid)
+            test_pid="$(cat $d/pid 2>/dev/null)"
             if [ "$pid" = "$test_pid" ]; then
                 rm $d/pid
                 for f in $(cat $d/access-files); do