cache-tree.c::cache_tree_find(): simplify internal API

Earlier cache_tree_find() needs to be called with a valid cache_tree,
but repeated look-up may find an invalid or missing cache_tree in between.
Help simplify the callers by returning NULL to mean "nothing appropriate
found" when the input is NULL.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -514,6 +514,8 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size)
 
 static struct cache_tree *cache_tree_find(struct cache_tree *it, const char *path)
 {
+	if (!it)
+		return NULL;
 	while (*path) {
 		const char *slash;
 		struct cache_tree_sub *sub;