refuse to save YMU759 songs

the .dmf format no longer allows YMU759 songs...

src/engine/engine.cpp

@@ -1007,6 +1007,13 @@ bool DivEngine::load(unsigned char* f, size_t slen) {
 }
 
 SafeWriter* DivEngine::save() {
+  // fail if this is an YMU759 song
+  if (song.system==DIV_SYSTEM_YMU759) {
+    logE("cannot save YMU759 song!\n");
+    lastError="YMU759 song saving is not supported";
+    return NULL;
+  }
+
   SafeWriter* w=new SafeWriter;
   w->init();
   // write magic

src/gui/gui.cpp

@@ -2383,6 +2383,11 @@ int FurnaceGUI::save(String path) {
     return 1;
   }
   SafeWriter* w=e->save();
+  if (w==NULL) {
+    lastError=e->getLastError();
+    fclose(outFile);
+    return 3;
+  }
 #ifdef FURNACE_ZLIB_COMPRESS
   unsigned char zbuf[131072];
   int ret;