[server] Fix crash in WMS server when... bad things happen

No idea what exactly the reason is, and it was only discovered by
countless hours of printf-debugging. So I'm just pushing the fix
for everyone else who might be affected.

src/server/qgswmsprojectparser.cpp

@@ -1146,6 +1146,11 @@ void QgsWMSProjectParser::addLayers( QDomDocument &doc,
         }
       }
 
+      if ( !ltGroup )
+      {
+        QgsDebugMsg( QString( "Skipping group %1, it could not be found" ).arg( name ) );
+        continue;
+      }
       QString shortName = ltGroup->customProperty( "wmsShortName" ).toString();
       QString title = ltGroup->customProperty( "wmsTitle" ).toString();