http: release strbuf on disabled alternates

This likely has no real-world impact on memory usage,
but it is cleaner for future readers.

Fixes: abcbdc03895f ("http: respect protocol.*.allow=user for http-alternates")
Signed-off-by: Eric Wong <e@80x24.org>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
