Passthrough is supposed to pass buffers through, so don't clip them
then, especially as the subclass might not even have called
gst_base_parse_set_frame_rate in passthrough mode to set up the
lead_in_ts and lead_out_ts private member variables. This is common,
as it's usually done in their handle_frame implementation, which
doesn't even get called in passthrough push mode.
As an extra safety, also don't do the buffer dropping if
gst_base_parse_set_frame_rate hasn't been called with proper data,
which can be checked based on whether there's a valid frame_duration.