tracing: Add NULL pointer check to trigger_data_free()

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

tracing: Add NULL pointer check to trigger_data_free()

If trigger_data_alloc() fails and returns NULL, event_hist_trigger_parse() jumps to the out_free error path. While kfree() safely handles a NULL pointer, trigger_data_free() does not. This causes a NULL pointer dereference in trigger_data_free() when evaluating data->cmd_ops->set_filter.

Fix the problem by adding a NULL pointer check to trigger_data_free().

The problem was found by an experimental code review agent based on gemini-3.1-pro while reviewing backports into v6.18.y.