uiomove: Add some assertions

Make sure that we don't try to copy with a negative resid.

Make sure that we don't walk off the end of the iovec array.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42098

(cherry picked from commit 8fd0ec53deaad34383d4b344714b74d67105b258)