sha1_file: release fallback base's memory in unpack_entry()

sha1_file: release fallback base's memory in unpack_entry()

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 <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>