git-write-tree should not crash if prefix does not exist

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-write-tree.c

@@ -52,6 +52,8 @@ int write_tree(unsigned char *sha1, int missing_ok, const char *prefix)
 	if (prefix) {
 		struct cache_tree *subtree =
 			cache_tree_find(active_cache_tree, prefix);
+		if (!subtree)
+			die("git-write-tree: prefix %s not found", prefix);
 		hashcpy(sha1, subtree->sha1);
 	}
 	else