refs: fix potential free of uninitialized variable

The `signature` variable in `git_reference_rename` isn't initialized and
neither does `git_reference__log_signature` always do. So if the latter
function fails, we'll call `git_signature_free` on this unininitialized
variable.

Fix the issue by initializing the pointer with `NULL`.

Author: pks-t

src/refs.c

@@ -606,7 +606,7 @@ int git_reference_rename(
 	const char *log_message)
 {
 	refs_update_head_payload payload;
-	git_signature *signature;
+	git_signature *signature = NULL;
 	git_repository *repo;
 	int error;