When a playing mp3 is deinitted, it's possible to reach get_buffer,
but all the internal pointers are NULL.  This would lead to a hard fault.
Avoid it by returning GET_BUFFER_ERROR instead.

There were several problems with the way this worked -- the read_count
approach was too complicated and I made a mistake "simplifying" it from
WaveFile.  And when the right channel was returned, it was off by 1 byte,
making it into static.

Instead, directly track which is the "other" channel that has data
available, and by using the right data type make the "+ channel"
arithmetic give the right result.

This requires a double cast (int16_t*)(void*) due to an alignment warning;
the alignment is now ensured manually, but the compiler doesn't make the
necessary inference that the low address bit must be clear.