tools/test/stress2/misc/all.debug.inc: skip undefined variables

On my ZFS based systems, no allocations occur with tags "newblk" or
"freework". This leads to errors executing the tests that check for
memory leaks. Skip the checks if the output of wmstat -m does not
contain lines corresponding to those allocations.

MFC after:	3 days

tools/test/stress2/misc/all.debug.inc

@@ -49,13 +49,15 @@ post_debug() {
 		debug_statfs=$debug_new; }
 
 	debug_new=`vmstat -m | grep -w freework | awk '{print $2}'`
-	[ $((debug_new - debug_freework)) -gt 0 -a $debug_new -gt 100 ] &&
+	[ -n "$debug_new" ] &&
+        [ $((debug_new - debug_freework)) -gt 0 -a $debug_new -gt 100 ] &&
 	    { printf "stress2: freework leak: %d/%d.\r\n" \
 	        $((debug_new - debug_freework)) $debug_new > $console
 		debug_freework=$debug_new; }
 
 	debug_new=`vmstat -m | grep -w newblk | awk '{print $2}'`
-	[ $((debug_new - debug_newblk)) -gt 0 -a $debug_new -gt 100 ] &&
+	[ -n "$debug_new" ] &&
+        [ $((debug_new - debug_newblk)) -gt 0 -a $debug_new -gt 100 ] &&
 	    { printf "stress2: newblk leak: %d/%d.\r\n" \
 	        $((debug_new - debug_newblk)) $debug_new > $console
 		debug_newblk=$debug_new; }