TCP Networking: When CONFIG_NET_TCP_WRITE_BUFF=y there is a situation where a...

TCP Networking:  When CONFIG_NET_TCP_WRITE_BUFF=y there is a situation where a NULL pointer may be dereferenced.  In this configuration, the TCP connection's 'semi-permnanent' callback, s_sndcb was nullified in tcp_close_disconnect.  However, other logic in tcp_lost_connection() attempt to use that callback reference after it was nullifed.  Fixed in tcp_lost_connectino() by adding a NULL pointer change before the access.  This was reported by  Dmitriy Linikov in Bitbucket Issue 72.