refs/reftable: fix leak when copying reflog fails

refs/reftable: fix leak when copying reflog fails

When copying a ref with the reftable backend we also copy the
corresponding log records. When seeking the first log record that we're
about to copy fails though we directly return from `write_copy_table()`
without doing any cleanup, leaking several allocated data structures.

Fix this by exiting via our common cleanup logic instead.

Reported-by: Jeff King <peff@peff.net> via Coverity
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>