pack-objects: re-validate data we copy from elsewhere.

pack-objects: re-validate data we copy from elsewhere.

When reusing data from an existing pack and from a new style
loose objects, we used to just copy it staight into the
resulting pack.  Instead make sure they are not corrupt, but
do so only when we are not streaming to stdout, in which case
the receiving end will do the validation either by unpacking
the stream or by constructing the .idx file.

Signed-off-by: Junio C Hamano <junkio@cox.net>