fix the handling of negative name query responses and the handling of

fix the handling of negative name query responses and the handling of
packets with no answer section in general.

The fix has 2 parts:

1) set ans_name to the name we queried if nmb->answers == NULL

2) check for nmb->answers == NULL in several other places where we
currently check for nmb->answers->data

While doing this, I noticed there are lots of places in our nmbd code
where we make assumptions about the packets being well formed. Someone
could easily implement a denial of service attack on nmbd by sending a
packet that causes a null pointer dereference. Does anyone feel like
going through the code and adding checks? Probably the best solution
is to have a single function that "validates" a packet, making sure
that all the required fields are there. This will be a bit tricky as
what fields are required varies a lot between packets. A first pass
would be a function that prints "SUSPECT PACKET" when it hits a packet
that it suspects does not have a required field (or the field is badly
formatted), then we could use this on a live system to find any cases
we've missed.

Any takers?