Prevent send-pack from segfaulting (backport from 'master')

4491e62ae9 (Prevent send-pack from
segfaulting when a branch doesn't match) is hereby cherry-picked
back to 'maint'.

If we can't find a source match, and we have no destination, we
need to abort the match function early before we try to match
the destination against the remote.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -504,8 +504,11 @@ static int match_explicit(struct ref *src, struct ref *dst,
 	if (!matched_src)
 		errs = 1;
 
-	if (dst_value == NULL)
+	if (!dst_value) {
+		if (!matched_src)
+			return errs;
 		dst_value = matched_src->name;
+	}
 
 	switch (count_refspec_match(dst_value, dst, &matched_dst)) {
 	case 1: