This fixes the following issue:
On the master channel insert the waveform scope before the fader.
Then set the meter position to custom and move the meter to the
very beginning of the chain.
Before this change, when set_meter_point() was called the
processor list only contained the Fader (_amp) and no other
processor. _main_outs was not yet present in the list, and
Route::maybe_note_meter_position() triggered an
and assert(_processor_after_last_custom_meter.lock());
See also d0dca7daf0
fd414ec158