fix crash triggered by port scans closing socket
fix crash triggered by port scans closing socket

nmap and nc port scanning closes the socket without
sshuttle noticing so it continues to try to use it,
with disaterous consequences. This work around traps
the disaster and allows the client to continue handling
other sockets.

mheiges/sshuttle,mheiges/sshuttle,mheiges/sshuttle