If a pack entry that's used as a delta base is corrupt, unpack_entry()
marks it as unusable and then searches the object again in the hope that
it can be found in another pack or in a loose file. The memory for this
external base object is never released. Free it after use.
Signed-off-by: Rene Scharfe <l.s.r@web.de> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>