media: iris: gen2: Add sanity check for session stop

In the Linux kernel, the following vulnerability has been resolved: media: iris: gen2: Add sanity check for session stop In iris_kill_session, inst->state is set to IRIS_INST_ERROR and session_close is executed, which will kfree(inst_hfi_gen2->packet). If stop_streaming is called afterward, it will cause a crash. Add a NULL check for inst_hfi_gen2->packet before sendling STOP packet to firmware to fix that.