WMS server: fix crashing context-based GetLegendGraphic

This code has been funded by Tuscany Region (Italy) - SITA (CIG: 6002233F59) and commissioned to Gis3W s.a.s.

The above statement applies also to the earlier commits:
5b38262 WMS server: advertise additional layer styles in GetCapabilities
95b4ebc WMS server: detect correct service URL even if REQUEST_URI is not defined
03edac9 WMS server: throw LayerNotDefined exception when undefined layer is requested
6e4c507 WMS server: support predefined layer styles in GetMap requests
4dead72 Allow multiple styles of layers in composer maps
50e77a9 Added support for renaming of map layer styles
8b5587f Improve the internals of map layer style manager
b4ad637 Make presets with styles working in composer
3fbb758 More reliable trigger of refresh when changing styles
803e822 Visibility presets also record layer's current style
603b3a5 Fixed problems with style switching not always working correctly
3ce9c70 Merge pull request #1755 from wonder-sk/multiple-styles
2144be0 Add Python bindings for map layer style manager
f00f4fd Style manager code cleanups
43d6042 Support for reading/writing additional styles for layers
19b7281 Fix cleanup of the style GUI utils singleton
edf3ebb Improve the Styles menu in legend context menu
547d018 Added a GUI utility class to support multiple styles per map layer
829f99b Initial core work on multiple styles per layer

Author: wonder-sk

src/server/qgswmsserver.cpp

@@ -1221,7 +1221,8 @@ QImage* QgsWMSServer::getMap( HitTest* hitTest )
   clearFeatureSelections( selectedLayerIdList );
 
   // QgsDebugMsg( "clearing filters" );
-  QgsMapLayerRegistry::instance()->removeAllMapLayers();
+  if ( !hitTest )
+    QgsMapLayerRegistry::instance()->removeAllMapLayers();
 
   //#ifdef QGISDEBUG
   //  theImage->save( QDir::tempPath() + QDir::separator() + "lastrender.png" );