maybe fix possible crash in low buffer sizes

src/engine/playback.cpp

@@ -1685,6 +1685,7 @@ void DivEngine::nextBuf(float** in, float** out, int inChans, int outChans, unsi
   totalProcessed=size-(runLeftG>>MASTER_CLOCK_PREC);
 
   for (int i=0; i<song.systemLen; i++) {
+    if (size>disCont[i].lastAvail) continue;
     disCont[i].fillBuf(disCont[i].runtotal,disCont[i].lastAvail,size-disCont[i].lastAvail);
   }