bcache: fix for gc crashing when no sectors are used

Signed-off-by: NNicholas Swenson <nks@daterainc.com>
Signed-off-by: NKent Overstreet <kmo@daterainc.com>

drivers/md/bcache/movinggc.c

@@ -184,7 +184,8 @@ static bool bucket_cmp(struct bucket *l, struct bucket *r)
 
 static unsigned bucket_heap_top(struct cache *ca)
 {
-	return GC_SECTORS_USED(heap_peek(&ca->heap));
+	struct bucket *b;
+	return (b = heap_peek(&ca->heap)) ? GC_SECTORS_USED(b) : 0;
 }
 
 void bch_moving_gc(struct cache_set *c)