test-tool: fix leak in delete-gpgsig command

test-tool: fix leak in delete-gpgsig command

We read the input into a strbuf, so we must free it. Without this, t1016
complains in SANITIZE=leak mode.

The bug was introduced in 7673ecd2dc (t1016-compatObjectFormat: add
tests to verify the conversion between objects, 2023-10-01). But nobody
seems to have noticed, probably because CI did not run these tests until
the fix in 6cd8369ef3 (t/lib-gpg: call prepare_gnupghome() in GPG2
prereq, 2024-07-03).

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