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>