Fix #2602: music doesn't change immediately on loading a save

(cherry-picked from commit 94b69e0281)

src/savegame.cpp

@@ -42,6 +42,7 @@
 #include "serialization/binary_or_text.hpp"
 #include "serialization/parser.hpp"
 #include "serialization/utf8_exception.hpp"
+#include "sound.hpp"
 #include "statistics.hpp"
 #include "version.hpp"
 #include "video.hpp"
@@ -204,6 +205,8 @@ bool loadgame::load_game()
 	// read classification to for loading the game_config config object.
 	gamestate_.classification() = game_classification(load_data_.load_config);
 
+	sound::clear_current_track();
+
 	if (skip_version_check) {
 		return true;
 	}

src/sound.cpp

@@ -576,6 +576,11 @@ void empty_playlist()
 	current_track_list.clear();
 }
 
+void clear_current_track()
+{
+	current_track.reset();
+}
+
 void play_music()
 {
 	if(!current_track) {

src/sound.hpp

@@ -53,6 +53,8 @@ void play_music_repeatedly(const std::string& id);
 void play_music_once(const std::string& id);
 // Empty the playlist
 void empty_playlist();
+// Clear current_track, allowing immediate music changes.
+void clear_current_track();
 // Start playing current music.
 void play_music();