Do *not* check if socket is closed by another thread. This is
race-condition prone, unnecessary, and harmful. VIO state was introduced
to debug the errors, not to change the behavior.

Rather than checking if socket is closed, add a DBUG_ASSERT that it is
*not* closed, because this is an actual logic error, and can potentially
lead to all sorts of funny behavior like writing error packets to Innodb
files.

Unlike closesocket(), shutdown(2) is not actually race-condition prone,
and it breaks poll() and read(), and it worked for longer than a decade,
and it does not need any state check in the code.