Evaluation: play "credits_silence" tune before "credits" to mask any trace of level music from demos

"level_nosync" was reactivating via P_Ticker from credits
demo and showing a music credit for a brief moment, where
there should not be any music credit.

src/k_credits.cpp

@@ -412,6 +412,7 @@ void F_StartCredits(void)
 	Music_StopAll();
 	S_StopSounds();
 
+	Music_Play("credits_silence"); // mask any music changes from demos
 	F_NewCreditsMusic("_creds", false);
 
 	F_CreditsReset();

src/music.cpp

@@ -164,6 +164,13 @@ void Music_Init(void)
 		tune.credit = false;
 	}
 
+	{
+		Tune& tune = g_tunes.insert("credits_silence");
+
+		tune.song = "";
+		tune.priority = 100;
+	}
+
 	{
 		Tune& tune = g_tunes.insert("credits");