AudioMixer: fill 0s at right place when no more buffers available

When mixer format is PCM_FLOAT and only a partial buffer available
at the end of playback, 0s can be filled at the beginning of the
mixer buffer which will overwrite the copied buffers, then glitch
is heard.

This is because OutPointer is not updated and the pointer always keeps
the start position of mixer buffer when mixer format is PCM_FLOAT.

Assign the value of 'fout' to 'out' to make sure 'out' holds the
current copy position.

CRs-Fixed: 872775

Change-Id: I50504c5a02eb0c69abfc9b047792b0f6f85b9ce8