Guard vector file writer against invalid layers

instead of crashing.

Fixes #21856

Funded by: QCooperative.net

src/core/qgsvectorfilewriter.cpp

@@ -2453,7 +2453,7 @@ QgsVectorFileWriter::SaveVectorOptions::SaveVectorOptions()
 
 QgsVectorFileWriter::WriterError QgsVectorFileWriter::prepareWriteAsVectorFormat( QgsVectorLayer *layer, const QgsVectorFileWriter::SaveVectorOptions &options, QgsVectorFileWriter::PreparedWriterDetails &details )
 {
-  if ( !layer )
+  if ( !layer || !layer->isValid() )
   {
     return ErrInvalidLayer;
   }