[lwip-members] accessing NULL pbuf in new code

[Jani Monoses]

Hi 
David I fixed two NULL pbuf accesses in new select() callback related code
They seem easy enough but might not be the best since I have not looked at the 
select
code as a whole and maybe some similar fixes are needed in other places where
p->tot_len is modified without checking whether it's a valid pbuf.
The crashes I got and fixed happen when recv_tcp and netconn_recv get NULL pbuf
which can happen when a FIN packet arrives.Simply CTRL-C ing a client which 
talked to the unix
simulator caused the latter to crash.
(In api_lib.c and api_msg.c )

Jani.