pack-objects: remove redundant and wrong call to deflateEnd()

pack-objects: remove redundant and wrong call to deflateEnd()

We somehow called deflateEnd() on a stream that we have called
deflateEnd() on already.

In fact, the second deflateEnd() has always been returning
Z_STREAM_ERROR.  We just never checked the error return from
that particular deflateEnd().

The first one returns 0 for success.  We might want to tighten
the check even more to check that.

Noticed by Marco.

Signed-off-by: Junio C Hamano <gitster@pobox.com>