wxGUI: check GRASS_PNGFILE first

(merge from devbr6, https://trac.osgeo.org/grass/changeset/33287)


git-svn-id: https://svn.osgeo.org/grass/grass/trunk@33288 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/gui_modules/render.py

@@ -175,7 +175,8 @@ class Layer(object):
         #
         # stop monitor
         #
-        del os.environ["GRASS_PNGFILE"]
+        if os.environ.has_key("GRASS_PNGFILE"):
+            del os.environ["GRASS_PNGFILE"]
         
         self.force_render = False