Fix use-after-free introduced in r300388.

In r300388, endnetconfig() was called on nc_handle which would release
the associated netconfig structure, which means tmpnconf->nc_netid
would be a use-after-free.

Solve this by doing endnetconfig() in return paths instead.

Reported by: jemalloc via kevlo
Reviewed by: cem, ngie (earlier version)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D11288