Add a missing com_unlock() to the serial port drain test code. This code

Add a missing com_unlock() to the serial port drain test code.  This code
is necessary to detect non-existant serial ports on machines (such as the
eMachines laptop) which don't have any serial port hardware at all, but
the missing com_unlock() caused systems to crash in certain situations for
obvious reasons.

Many thanks to Glenn Johnson <glennpj@charter.net> for keeping on my back
about the issue and for testing the fix.  This should also fix the last of the
serial-port related bug reports stemming from the original 1.0A release.