BUG/MINOR: backend: check srv_conn before dereferencing it

Commit 3c4e19f42 ("BUG/MEDIUM: backend: always release the previous
connection into its own target srv_list") introduced a valid warning
about a null-deref risk since we didn't check conn_new()'s return value.

This patch must be backported to 1.9 with the patch above.
diff --git a/src/backend.c b/src/backend.c
index 4340ba5..eac4154 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1343,7 +1343,8 @@
 	/* no reuse or failed to reuse the connection above, pick a new one */
 	if (!srv_conn) {
 		srv_conn = conn_new();
-		srv_conn->target = s->target;
+		if (srv_conn)
+			srv_conn->target = s->target;
 		srv_cs = NULL;
 	}