BUG/MINOR: http: url32+src should check cli_conn before using it

In function smp_fetch_url32_src(), it's better to check the value of
cli_conn before we go any further.

This patch needs to be backported to 1.6 and 1.5.
(cherry picked from commit db5af61f3cae7f937b56994c0b7c153f7ceeb366)
(cherry picked from commit 9fb2e8d874a44e820fad5e7bc4de39d0775a6dd8)
diff --git a/src/proto_http.c b/src/proto_http.c
index ba14e7e..c55258a 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -11223,6 +11223,9 @@
 	struct chunk *temp;
 	struct connection *cli_conn = objt_conn(l4->si[0].end);
 
+	if (!cli_conn)
+		return 0;
+
 	if (!smp_fetch_url32(px, l4, l7, opt, args, smp, kw))
 		return 0;