ptrace: Do not pass a negative resid to proc_rwmem()

While here, avoid truncting uio_resid in proc_rwmem().

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49479

(cherry picked from commit 1a80a157cbe823ba75bb88823dbf1b245fe87c99)