stChannelState::Echo[3] was uninitialized prior to this fix.
stChannelState::Echo is read by CFamiTrackerDoc::RetrieveSoundState(),
a strange non-const function called upon recalling or retrieving
channel state (eg. beginning playback). The algorithm is complex
and I don't understand it. I'm just "fixing" it.

The algorithm is strange and complex

m_iFrame - other.m_iFrame is intended to produce a negative result.
In the old code, it produced a very large unsigned integer (bad),
but it got casted to a negative signed integer upon * or %,
so the code was merely unclear but not wrong.
Anyway I've cleaned it up (at the cost of verbosity).