transport-helper: fix strbuf leak in push_refs_with_push()

transport-helper: fix strbuf leak in push_refs_with_push()

We loop over the refs to push, building up a strbuf with the set of
"push" directives to send to the remote helper. But if the atomic-push
flag is set and we hit a rejected ref, we'll bail from the function
early. We clean up most things, but forgot to release the strbuf.

Fixing this lets us mark t5541 as leak-free.

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