Fixed PSG sound which would sometimes not play (regression).

By the way, SPU.cpp should be cleaned up. Some parts are messy and it takes hours to get compiled.

desmume/src/SPU.cpp

@@ -510,11 +510,14 @@ void SPU_struct::KeyOn(int channel)
 	default: break;
 	}
 
+	if(thischan.format != 3)
+	{
 		if(thischan.double_totlength_shifted == 0)
 		{
 			printf("INFO: Stopping channel %d due to zero length\n",channel);
 			thischan.status = CHANSTAT_STOPPED;
 		}
+	}
 	
 	thischan.double_totlength_shifted = (double)(thischan.totlength << format_shift[thischan.format]);
 }