Fixes segmentation fault

Author: pblottiere

src/core/qgsogcutils.cpp

@@ -2191,7 +2191,10 @@ QDomElement QgsOgcUtils::expressionToOgcExpression( const QgsExpression &exp,
       break;
     }
     default:
-      *errorMessage = QObject::tr( "Node type not supported in expression translation: %1" ).arg( node->nodeType() );
+    {
+      if ( errorMessage )
+        *errorMessage = QObject::tr( "Node type not supported in expression translation: %1" ).arg( node->nodeType() );
+    }
   }
   // got an error
   return QDomElement();