sparse-checkout: consolidate cleanup when writing patterns

sparse-checkout: consolidate cleanup when writing patterns

In write_patterns_and_update(), we always need to free the pattern list
before exiting the function.  Rather than handling it manually when we
return early, we can jump to an "out" label where cleanup happens. This
let us drop one line, but also establishes a pattern we can use for
other cleanup.

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