BUG/MAJOR: stick_table: Complete incomplete SEGV fix

This commit completes the incomplete segmentation fault fix
in commit ac1f3ed64b.

Likewise it must be backported to haproxy 1.8.

reg-tests/stick-table/h00000.vtc

@@ -0,0 +1,30 @@
+# Shipped with the commit fixing the bug.
+
+varnishtest "Stick Table: Crash when accessing unknown key."
+feature ignore_unknown_macro
+
+server s0 {
+	rxreq
+	txresp
+} -start
+
+haproxy h0 -conf {
+	defaults
+		timeout connect 5000ms
+		timeout client 50000ms
+		timeout server 50000ms
+
+	frontend test
+		mode http
+		bind "fd@${fe1}"
+		stick-table type ip size 1m expire 1h store gpc0
+		http-request deny if { src,table_trackers(test) eq 1 }
+		http-request deny if { src,in_table(test) }
+		http-request deny deny_status 200
+} -start
+
+client c0 -connect ${h0_fe1_sock} {
+	txreq -url "/"
+	rxresp
+	expect resp.status == 200
+} -run

src/stick_table.c

@@ -1596,7 +1596,9 @@ static int sample_conv_table_trackers(const struct arg *arg_p, struct sample *sm
 	smp->data.type = SMP_T_SINT;
 	smp->data.u.sint = 0;
 
-	if (ts)
+	if (!ts)
+		return 1;
+
 	smp->data.u.sint = ts->ref_cnt;
 
 	stktable_release(t, ts);