app/testpmd: fix forward ports update

app/testpmd: fix forward ports update

When the forward ports are changed either by new portlist\portmask
configurations or by a port detachment, all the old forward streams
are freed and new streams are allocated to be aligned with the new
forward ports.

If the number of the forward ports drops to 0, there is an attempt
to wrongly allocate 0 memory for the streams.

Skip the streams memory allocation if no forward ports are configured.

Fixes: ce8d561418d4 ("app/testpmd: add port configuration settings")
Cc: stable@dpdk.org
Signed-off-by: Matan Azrad <matan@mellanox.com>