Fix missing quotes for possibly unresolved variables in shell test-statements

Related: OS#5736

Change-Id: I7e7f4f95d1a7cc886360c733908c63a2843b0906
diff --git a/ttcn3-dumpcap-start.sh b/ttcn3-dumpcap-start.sh
index 3aec9b9..eb390fd 100755
--- a/ttcn3-dumpcap-start.sh
+++ b/ttcn3-dumpcap-start.sh
@@ -30,7 +30,7 @@
 kill_rm_pidfile $PIDFILE_NETCAT
 kill_rm_pidfile $PIDFILE_PCAP
 
-if [ ! -x $DUMPCAP ]; then
+if [ ! -x "$DUMPCAP" ]; then
 	echo "Missing required dumpcap binary at ${DUMPCAP}"
 	exit 31
 fi