tap: handle null namespace and bridge correctly

tap: handle null namespace and bridge correctly

Type: fix

In tap_create_if(), if args->host_namespace or args->host_bridge are
null because no values were set for those, the virtio_if_t entry in
virtio_main.interfaces ends up getting populated with values of "(nil)"
in net_ns or host_bridge, respectively.

Check whether args->host_namespace and args->host_bridge are null before
trying to set the corresponding fields on virtio_if_t.

Change-Id: I8e1e66a6d7b246e7c66fece406d116ffb1312c64
Signed-off-by: Matthew Smith <mgsmith@netgate.com>