resolve_ref(): verify that the input refname has the right format

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/refs.c b/refs.c
index b055501..ee3e0cc 100644
--- a/refs.c
+++ b/refs.c
@@ -504,6 +504,9 @@
 	if (flag)
 		*flag = 0;
 
+	if (check_refname_format(ref, REFNAME_ALLOW_ONELEVEL))
+		return NULL;
+
 	for (;;) {
 		char path[PATH_MAX];
 		struct stat st;