refs: prevent accidental NULL dereference in write_pseudoref

Several of the refs functions take NULL to indicate that the ref is not
to be updated.  If refs_update_ref were called with a NULL new object
ID, we could pass that NULL pointer to write_pseudoref, which would then
segfault when it dereferenced it.  Instead, simply return successfully,
since if we don't want to update the pseudoref, there's nothing to do.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -583,6 +583,9 @@ static int write_pseudoref(const char *pseudoref, const struct object_id *oid,
 	struct strbuf buf = STRBUF_INIT;
 	int ret = -1;
 
+	if (!oid)
+		return 0;
+
 	strbuf_addf(&buf, "%s\n", oid_to_hex(oid));
 
 	filename = git_path("%s", pseudoref);