bcache: fix incorrect pointer check

With commit b44db5d1a7
needs to check allocated pointer for failed malloc().

Existing check was actually no checking anything so failing
malloc here would result in segfault (although with very
low chance to ever happen).

lib/device/bcache.c

@@ -74,7 +74,7 @@ static struct cb_set *_cb_set_create(unsigned nr)
 	unsigned i;
 	struct cb_set *cbs = malloc(sizeof(*cbs) + nr * sizeof(*cbs->vec));
 
-	if (!cbs->vec)
+	if (!cbs)
 		return NULL;
 
 	dm_list_init(&cbs->free);