ifnet: Assert that we are assigning network stack correctly

It makes no sense to assign NULL vnet to an interface when the kernel
option VIMAGE is enabled. Add an assertion to catch that.

This will also help diagnosing problem report [1] and [2].

1. https://bugs.freebsd.org/275381
2. https://bugs.freebsd.org/282168

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46982

(cherry picked from commit d1d839d0b593541174ca48c675c9eff4ddb4715e)
(cherry picked from commit 2f3c87296cc727fc21fbb4d549deaed7dfa2fef1)