bufferevent_pair: fix infer warnings due to upcast() return NULL

The only user that is care about this is bufferevent_pair_get_partner(),
so let's move the check there.

Author: azat

bufferevent_pair.c

@@ -56,8 +56,6 @@ static inline struct bufferevent_pair *
 upcast(struct bufferevent *bev)
 {
 	struct bufferevent_pair *bev_p;
-	if (!BEV_IS_PAIR(bev))
-		return NULL;
 	bev_p = EVUTIL_UPCAST(bev, struct bufferevent_pair, bev.bev);
 	EVUTIL_ASSERT(BEV_IS_PAIR(&bev_p->bev.bev));
 	return bev_p;
@@ -341,10 +339,9 @@ bufferevent_pair_get_partner(struct bufferevent *bev)
 {
 	struct bufferevent_pair *bev_p;
 	struct bufferevent *partner = NULL;
-	bev_p = upcast(bev);
-	if (! bev_p)
+	if (!BEV_IS_PAIR(bev))
 		return NULL;
-
+	bev_p = upcast(bev);
 	incref_and_lock(bev);
 	if (bev_p->partner)
 		partner = downcast(bev_p->partner);