harness: Make sure pad functions are not called after teardown

harness: Make sure pad functions are not called after teardown

For the query function there's a risk that the function may be called
after the harness has been teared down. Since the function accesses a
pointer to the harness via the pad's data, the harness must protect
itself against this.

Event and chain function is also handled for constistency, although
they don't have the same problem since the gstpad.c checks whether the
pad is flushing before calling these.