wxGUI: fix raster querying broken https://trac.osgeo.org/grass/changeset/67751 (https://trac.osgeo.org/grass/ticket/2920)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@67838 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/mapdisp/frame.py

@@ -916,7 +916,7 @@ class MapFrame(SingleMapFrame):
         self._QueryMapDone()
 
         self._highlighter_layer.Clear()
-        if 'Category' in vectQuery[0]:
+        if vectQuery and 'Category' in vectQuery[0]:
             self._queryHighlight(vectQuery)
 
         result = rastQuery + vectQuery