midx.c: clean up chunkfile after reading the MIDX

In order to read the contents of a MIDX, we initialize a chunkfile structure which can read the table of contents and assign pointers into different sections of the file for us. We do call free(), since the chunkfile struct is heap allocated, but not the more appropriate free_chunkfile(), which also frees memory that the structure itself owns. Call that instead to avoid leaking memory in this function. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>