net/mlx5: Fix NULL pointer dereference in extended destination

The cited commit refactored the encap id into a struct pointed from the
destination.
Bug fix for the case there is no encap for one of the destinations.

Fixes: 2b688ea5 ("net/mlx5: Add flow steering actions to fs_cmd shim layer")
Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>