Handle read errors in Protocols.LysKOM.Raw.

If there is a network outage, con->read() will return 0 (not the empty
string) after a (long) while.  Treat that properly by calling
connection_lost() in that case as well.

Fixed a backrace in conwrite by returning 0 if the connection
is closed.

(cherry picked from commit 7cdea47d8c28017dbdbff49607c811ca0472000c)