BUILD: http: address a couple of null-deref warnings at -Wextra

These two warnings are caused by the use of objt_server() without
checking its result. These are turned to __objt_server() which is
safe there.
diff --git a/src/proto_http.c b/src/proto_http.c
index 22eab8b..c17d67ca 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -500,7 +500,7 @@
 	trash.data = strlen(HTTP_302);
 	memcpy(trash.area, HTTP_302, trash.data);
 
-	srv = objt_server(s->target);
+	srv = __objt_server(s->target);
 
 	/* 2: add the server's prefix */
 	if (trash.data + srv->rdr_len > trash.size)
@@ -5367,7 +5367,7 @@
 			goto return_bad_resp;
 
 		txn->flags &= ~TX_SCK_MASK;
-		if (objt_server(s->target)->cookie && (s->flags & SF_DIRECT))
+		if (__objt_server(s->target)->cookie && (s->flags & SF_DIRECT))
 			/* the server did not change, only the date was updated */
 			txn->flags |= TX_SCK_UPDATED;
 		else