tag: die when listing missing or corrupt objects

We don't usually bother looking at tagged objects at all
when listing. However, if "-n" is specified, we open the
objects to read the annotations of the tags.  If we fail to
read an object, or if the object has zero length, we simply
silently return.

The first case is an indication of a broken or corrupt repo,
and we should notify the user of the error.

The second case is OK to silently ignore; however, the
existing code leaked the buffer returned by read_sha1_file.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -93,8 +93,12 @@ static void show_tag_lines(const unsigned char *sha1, int lines)
 	size_t len;
 
 	buf = read_sha1_file(sha1, &type, &size);
-	if (!buf || !size)
+	if (!buf)
+		die_errno("unable to read object %s", sha1_to_hex(sha1));
+	if (!size) {
+		free(buf);
 		return;
+	}
 
 	/* skip header */
 	sp = strstr(buf, "\n\n");