PPageExternalFilters: Cosmetics

* Remove an unneeded call to EnableToolTips.
* Add some asserts.
MpaDecFilter: Fix MPC-HC compilation with ffmpeg audio disabled

Lite builds were unaffected, but if you built a "normal" MPC-HC
with ffmpeg audio decoders disabled, it would not find
m_FFAudioDec, as it is only around when ffmpeg audio decoders are
enabled.