read_object_with_reference: don't read beyond the buffer

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -1943,7 +1943,8 @@ void *read_object_with_reference(const unsigned char *sha1,
 		}
 		ref_length = strlen(ref_type);
 
-		if (memcmp(buffer, ref_type, ref_length) ||
+		if (ref_length + 40 > isize ||
+		    memcmp(buffer, ref_type, ref_length) ||
 		    get_sha1_hex((char *) buffer + ref_length, actual_sha1)) {
 			free(buffer);
 			return NULL;