Merge branch 'fix_async_write_png' into 'master'

Avoid starting async png writing when overlay image is null (#6638)

Closes #6638

See merge request OpenMW/openmw!1679

apps/openmw/mwrender/globalmap.cpp

@@ -635,6 +635,8 @@ namespace MWRender
 
     void GlobalMap::asyncWritePng()
     {
+        if (mOverlayImage == nullptr)
+            return;
         // Use deep copy to avoid any sychronization
         mWritePng = new WritePng(new osg::Image(*mOverlayImage, osg::CopyOp::DEEP_COPY_ALL));
         mWorkQueue->addWorkItem(mWritePng, /*front=*/true);