removed warning if qstat is not found

qstat can not be found on host with outdated debian versions. These hosts do not have GPU's installed.

prolog.d/10-GPU_allocate.sh

@@ -35,7 +35,7 @@ function clean_up() {
 
 ### get requested number of GPU's
 # use hard resource list first
-NGPUS=$(qstat -j ${JOB_ID} | sed -n "s/hard resource_list:.*gpu=\([[:digit:]]\+\).*/\1/p")
+NGPUS=$(qstat -j ${JOB_ID} 2>/dev/null | sed -n "s/hard resource_list:.*gpu=\([[:digit:]]\+\).*/\1/p")
 # set NGPUS to zero if empty
 if [ -z "${NGPUS}" ] ; then
     NGPUS=0