virtio-serial: Fix check for 'assert'; prevent NULL derefs

In the flush_queued_data() function, we expect port to be valid. Assert
only for port and not port || discard.

Reported-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>