nan when no backdrop for painting

[danx0r]

reported by danh:

when dragging a painting to the sky through a door I got
File "/home/daniel/museum/liboh/scripts/ironpython/test.py", line 374,
in processRPC
self.reallyProcessRPC(header,name,arg)
File "/home/daniel/museum/liboh/scripts/ironpython/test.py", line 175,
in reallyProcessRPC
if not painting in self.objects:
ValueError: invalid literal for float(): nan

[danx0r]

not fatal as long as the scene is closed up (leaves no place for sky)