virtio-serial: fix segfault on NULL port names

Commit d0a0bfe6729ef6044d76ea49fafa07e29fa598bd added checks for port
names, but didn't add a check to ensure port->name is non-NULL.  This
results in a SIGSEGV when adding a port when one of the previously-added
ports didn't have the 'name' property set.

https://bugzilla.redhat.com/show_bug.cgi?id=1192775

Reported-by: vivian zhang <vivianzhang@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
