Shutdown memory error message when using "Dummy"

[M374LX]

As said in #2376, when using the "Dummy" audio device, the following error message is show at the program's shutdown:
*** Error in lmms': free(): invalid size: 0x00000000020169b0 ***`

Here is what happens, which may be related to this problem:

1. By calling Mixer::stopProcessing(), the FIFO writer is stopped and deleted (audio devices, as opposed to audio file devices, typically use Mixer with the FIFO writer).
2. The audio device (Dummy), still running, calls Mixer::nextBuffer() which now renders a new buffer without the FIFO writer, but it looks like something goes wrong.

[zonkmachine]

This is fixed in #2793