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

In the Linux kernel, the following vulnerability has been resolved: 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.