When loading a sound effect, the engine would deallocate the memory for the
currently playing sound effect, and stop playback afterwards. This commit
changes this to stop playback first, and then deallocate and load the new
sound effect.
This should fix the problem with the mixer accessing deallocated memory
reported in issue #12852.
The first name is Janusz, not Janus.
The correct name was used in AUTHORS and credits.
In case of doubts, see his personal webpage: https://www.jbw.pl/ - name is in the page footer