[BUGFIX][QGIS Server] Do not cache invalid layer

After readLayerXml, the server stored layers in cache and in registry without verifying validity.

Author: rldhont

src/server/qgshostedrdsbuilder.cpp

@@ -61,7 +61,7 @@ QgsMapLayer* QgsHostedRDSBuilder::createMapLayer( const QDomElement& elem,
     {
       rl = qobject_cast<QgsRasterLayer*>( QgsMSLayerCache::instance()->searchLayer( uri, layerName ) );
     }
-    if ( !rl )
+    if ( !rl || !rl->isValid() )
     {
       QgsDebugMsg( "hostedrds layer not in cache, so create and insert it" );
       rl = new QgsRasterLayer( uri, layerNameFromUri( uri ) );

src/server/qgsserverprojectparser.cpp

@@ -278,6 +278,10 @@ QgsMapLayer* QgsServerProjectParser::createLayerFromElement( const QDomElement&
     layer->readLayerXML( const_cast<QDomElement&>( elem ) ); //should be changed to const in QgsMapLayer
     //layer->setLayerName( layerName( elem ) );
 
+    if ( !layer->isValid() )
+    {
+      return nullptr;
+    }
     // Insert layer in registry and cache before addValueRelationLayersForLayer
     if ( !QgsMapLayerRegistry::instance()->mapLayer( id ) )
       QgsMapLayerRegistry::instance()->addMapLayer( layer, false, false );