fix crash caused by calling Mix_FadeOutChannel with 0 as the second parameter

src/audio/audio.cpp

@@ -91,7 +91,6 @@ namespace gdw {
 		if (!sound_ptr) {
 			log(log_level::error) << "no sound " << name << " found in audio collection" << std::endl;
 		}
-
 		int channel = Mix_PlayChannel(-1, sound_ptr->get(), loop);
 		if (channel < 0) {
 			audio_error("couldn't play sound");
@@ -104,7 +103,7 @@ namespace gdw {
 		if(channel < -1) {
 			return;
 		}
-		Mix_FadeOutChannel(channel, fade_out);
+		Mix_FadeOutChannel(channel, fade_out + 1);
 	}
 
 	void audio::stereo(int channel, int left, int right) {