app/testpmd: fix IP next proto in set raw decap/encap

app/testpmd: fix IP next proto in set raw decap/encap

The IP's next protocol will be specified by user when set IP-in-IP
tunnel header via set raw decap/encap commands.

Currently this field is wrongly set to zero if there is no upper layer.
This leads to the encapsulated IP-in-IP tunnel header is not correct.

This next protocol field should be leave it as-is if there is no upper
layer or value is already set.

Fixes: 30626def03d6 ("app/testpmd: support raw encap/decap actions")
Cc: stable@dpdk.org
Signed-off-by: Xiaoyu Min <jackmin@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>