fetch: do not create ref from empty name

Previously, the refspec "<src>:" would be expanded to
"<src>:refs/heads/". Instead, treat an empty <dst> just like refspecs
without a colon.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/remote.c b/remote.c
index d66e2f3..39583d5 100644
--- a/remote.c
+++ b/remote.c
@@ -1254,7 +1254,7 @@
 
 static struct ref *get_local_ref(const char *name)
 {
-	if (!name)
+	if (!name || name[0] == '\0')
 		return NULL;
 
 	if (!prefixcmp(name, "refs/"))