refs/files: fix NULL pointer deref when releasing ref store

The `free_ref_cache()` function is not `NULL` safe and will thus
segfault when being passed such a pointer. This can easily happen when
trying to release a partially initialized "files" ref store. Fix this.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs/ref-cache.c

@@ -71,6 +71,8 @@ static void free_ref_entry(struct ref_entry *entry)
 
 void free_ref_cache(struct ref_cache *cache)
 {
+	if (!cache)
+		return;
 	free_ref_entry(cache->root);
 	free(cache);
 }