net: mscc: ocelot: Fix crash when adding interface under a lag

net: mscc: ocelot: Fix crash when adding interface under a lag

Commit 15faa1f67ab4 ("lan966x: Fix crash when adding interface under a lag")
fixed a similar issue in the lan966x driver caused by a NULL pointer dereference.
The ocelot_set_aggr_pgids() function in the ocelot driver has similar logic
and is susceptible to the same crash.

This issue specifically affects the ocelot_vsc7514.c frontend, which leaves
unused ports as NULL pointers. The felix_vsc9959.c frontend is unaffected as
it uses the DSA framework which registers all ports.

Fix this by checking if the port pointer is valid before accessing it.

Fixes: 528d3f190c98 ("net: mscc: ocelot: drop the use of the "lags" array")
Signed-off-by: Jerry Wu <w.7erry@foxmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/tencent_75EF812B305E26B0869C673DD1160866C90A@qq.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>