parse_tag_buffer(): do not prefixcmp() out of range

There is a check (size < 64) at the beginning of the function, but
that only covers object+type lines.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tag.c

diff --git a/tag.c b/tag.c
index ecf7c1e..7d38cc0 100644
--- a/tag.c
+++ b/tag.c
@@ -97,7 +97,9 @@
 		item->tagged = NULL;
 	}
 
-	if (prefixcmp(bufptr, "tag "))
+	if (bufptr + 4 < tail && !prefixcmp(bufptr, "tag "))
+		; 		/* good */
+	else
 		return -1;
 	bufptr += 4;
 	nl = memchr(bufptr, '\n', tail - bufptr);
@@ -106,7 +108,7 @@
 	item->tag = xmemdupz(bufptr, nl - bufptr);
 	bufptr = nl + 1;
 
-	if (!prefixcmp(bufptr, "tagger "))
+	if (bufptr + 7 < tail && !prefixcmp(bufptr, "tagger "))
 		item->date = parse_tag_date(bufptr, tail);
 	else
 		item->date = 0;