mnl: catch bogus expressions before crashing

mnl: catch bogus expressions before crashing

We can't recover from errors here, but we can abort with a more
precise reason than 'segmentation fault', or stack corruptions
that get caught way later, or not at all.

expr->value is going to be read, we can't cope with other expression
types here.

We will copy to stack buffer of IFNAMSIZ size, abort if we would
overflow.

Check there is a NUL byte present too.
This is a preemptive patch, I've seen one crash in this area but
no reproducer yet.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>