NAT44 segv on unknown proto on inside interface

NAT44 segv on unknown proto on inside interface

When a packet with an unknown proto arrives
on an inside interface and there are no existing sessions
for the source address, a segv occurs.

snat_in2out_unknown_proto() finds the head of the sessions
dlist, fetches the address of the next element using
head->next, and then dereferences the next element. On the
first packet received from a source address, head->next is
~0, so this results in a segv.

Check that the session list is not empty before trying to
traverse it.

Also removed unnecessary lookup against tsm->user_hash.
Prior call to nat_user_get_or_create() already performed
that lookup and added a user if one didn't exist.

Change-Id: If73e79aa2f8e3962ab7b876ecf55aea40d7a5472
Signed-off-by: Matthew Smith <mgsmith@netgate.com>