Fix address annotation in xml output from w

The libxo xml feature of adding an annotation with the "original"
address from the utmpx file if it is different than the final "from"
field was broken by r351379. This was pointed out by the gcc error
that save_p might be used uninitialized. Save the original address
as needed in each entry, don't just use the last one from the previous
loop.

Reviewed by: marcel@
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21390