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

Related: OS#5736

Change-Id: I7e7f4f95d1a7cc886360c733908c63a2843b0906
diff --git a/ttcn3-tcpdump-start.sh b/ttcn3-tcpdump-start.sh
index 0ce07cd..8433c80 100755
--- a/ttcn3-tcpdump-start.sh
+++ b/ttcn3-tcpdump-start.sh
@@ -35,7 +35,7 @@
 	CMD="sudo $TCPDUMP -U"
 fi
 
-if [ -x $DUMPCAP ]; then
+if [ -x "$DUMPCAP" ]; then
     CAP_ERR="1"
     if [ -x /sbin/setcap ]; then
 	# N. B: this check requires libcap2-bin package