routing: fix crash when RTM_CHANGE results in no-op for the multipath
route.

Reporting logic assumed there is always some nhop change for every
successful modification operation. Explicitly check that the changed
nexthop indeed exists when reporting back to userland.

MFC after: 2 weeks
Reported by: Claudio Jeker <claudio.jeker@klarasystems.com>
Tested by: Claudio Jeker <claudio.jeker@klarasystems.com>