Fix: extmidi did not move on to next song after playing ends. (#11469)

`song` is no longer a C-style string so cannot be cleared by writing a NUL char.
Use `.clear()` to properly clear a std::string.

src/music/extmidi.cpp

@@ -125,7 +125,7 @@ void MusicDriver_ExtMidi::DoPlay()
 			FALLTHROUGH;
 
 		default:
-			this->song[0] = '\0';
+			this->song.clear();
 			break;
 	}
 }