Fix nice fade variables persisting when they shouldn't

This fixes a regression from 2.3 where the very beginning of A New
Dimension isn't silent.

A New Dimension's level music is set to Predestined Fate, but there is a
script box the player touches right upon spawning that stops playing
music. Then after the player ascends upwards, they touch a script box
that plays Predestined Fate. But in 2.2 and before, the very beginning
is silent due to the script box that stops music.

However in 2.3, due to the changes made to playing music during a fade,
the initial level music trying to play Predestined Fate during a music
fadeout from the main menu resulted in Predestined Fate being stored in
the nice fade variables, which kicked in after halting the music since
halting didn't reset those variables.

This resets those variables whenever music is halted, and now the
beginning of A New Dimension is back to how it was in 2.2 and before.

desktop_version/src/Music.cpp

@@ -1032,6 +1032,8 @@ void musicclass::haltdasmusik(void)
     currentsong = -1;
     m_doFadeInVol = false;
     m_doFadeOutVol = false;
+    nicefade = false;
+    nicechange = -1;
 }
 
 void musicclass::silencedasmusik(void)