send-pack: fix leaking push cert nonce

send-pack: fix leaking push cert nonce

When retrieving the push cert nonce from the server, we first store the
constant returned by `server_feature_value()` and then, if the nonce is
valid, we duplicate the nonce memory to a NUL-terminated string, so that
we can pass it to `generate_push_cert()`. We never free the latter and
thus cause a memory leak.

Fix this by storing the limited-lifetime nonce into a scope-local
variable such that the long-lived, allocated nonce can be easily freed
without having to cast away its constness.

This leak was exposed by t5534, but fixing it is not sufficient to make
the whole test suite leak free.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>