Fix a use after free in update_rtm_from_rc().

update_rtm_from_rc() calls update_rtm_from_info() internally.
The latter one may update provided prtm pointer with a new rtm.
Reassign rtm from prtm afeter calling update_rtm_from_info() to
avoid touching the freed rtm.

PR: 255871
Submitted by: lylgood@foxmail.com
MFC after: 3 days