fix crash

Author: jef-n

src/core/symbology-ng/qgssymbolv2.cpp

@@ -89,6 +89,8 @@ QgsMapUnitScale QgsSymbolV2::mapUnitScale() const
   }
 
   QgsSymbolLayerV2List::const_iterator it = mLayers.constBegin();
+  if( it == mLayers.constEnd() )
+    return QgsMapUnitScale();
 
   QgsMapUnitScale scale = ( *it )->mapUnitScale();
   ++it;