app/testpmd: fix port id check in Tx VLAN command

app/testpmd: fix port id check in Tx VLAN command

To set Tx vlan offloads, it is required to stop port firstly. But before
checking whether the port is stopped, the port id entered by the user
is not checked for validity. When the port id is illegal, it would lead
to a segmentation fault since it attempts to access a member of
non-existent port.

This patch adds verification of port id in tx vlan offloads and remove
duplicated check.

Fixes: 597f9fafe13b ("app/testpmd: convert to new Tx offloads API")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>