packfile: ensure `close_pack_revindex()` frees in-memory revindex

packfile: ensure `close_pack_revindex()` frees in-memory revindex

The following commit will introduce a case where we write a MIDX bitmap
over packs that do not themselves have on-disk *.rev files.

This case is supported within Git, and we will simply fall back to
generating the revindex in memory. But we don't ever release that
memory, causing a leak that is exposed by a test introduced in the
following commit.

(As far as I could find, we never free()'d memory allocated as a
byproduct of creating an in-memory revindex, likely because that code
predates the leak-checking niceties we have in the test suite now.)

Rectify this by calling `FREE_AND_NULL()` on the `p->revindex` field
when calling `close_pack_revindex()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>