[NETLABEL]: Fix NULL deref in netlbl_unlabel_staticlist_gen() if ifindex not found

dev_get_by_index() may return NULL if nothing is found. In 
net/netlabel/netlabel_unlabeled.c::netlbl_unlabel_staticlist_gen() the 
function is called, but the return value is never checked. If it returns 
NULL then we'll deref a NULL pointer on the very next line.
I checked the callers, and I don't think this can actually happen today, 
but code changes over time and in the future it might happen and it does 
no harm to be defensive and check for the failure, so that if/when it 
happens we'll fail gracefully instead of crashing.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Acked-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>