pack-bitmap.c: don't leak type-level bitmaps

pack-bitmap.c: don't leak type-level bitmaps

test_bitmap_walk() is used to implement `git rev-list --test-bitmap`,
which compares the result of the on-disk bitmaps with ones generated
on-the-fly during a revision walk.

In fa95666a40 (pack-bitmap.c: harden 'test_bitmap_walk()' to check type
bitmaps, 2021-08-24), we hardened those tests to also check the four
special type-level bitmaps, but never freed those bitmaps. We should
have, since each required an allocation when we EWAH-decompressed them.

Free those, plugging that leak, and also free the base (the scratch-pad
bitmap), too.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>