Don't pretend nm_*alloc and nm_strdup can return NULL

Since error handling wasn't implemented all the way for memory
allocations, nm_*-versions of allocators were implemented, guaranteeded
to return a valid pointer or not return at all. Since they can't return
NULL, we shouldn't pretend they do.

Lot of places were cleaned up already, but here are a couple more.

Signed-off-by: Max Sikstrom <msikstrom@op5.com>