Previously, the function _channelMessage took two data bytes, even
though the AfterTouch and ProgramChange messages only have a single data
byte. At the only callsite of _channelMessage, this caused us to always
index into the raw message for both data bytes. This would panic with
index out of range for AfterTouch and ProgramChange messages, since the
raw message is shorter.

To fix this, we pass the raw data message into _channelMessage as a byte
slice and only index into the second data byte for the messages that use
both data bytes.

Bugfix: fix panic in ListenTo for some msg types

Closes #55

See merge request gomidi/midi!8