reftable: introduce `REFTABLE_FREE_AND_NULL()`

reftable: introduce `REFTABLE_FREE_AND_NULL()`

We have several calls to `FREE_AND_NULL()` in the reftable library,
which of course uses free(3P). As the reftable allocators are pluggable
we should rather call the reftable specific function, which is
`reftable_free()`.

Introduce a new macro `REFTABLE_FREE_AND_NULL()` and adapt the callsites
accordingly.

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