I noticed that I missed the fact that `cimg` is used in the second `imshow()` call. Changed the scope of the second function call to be within the if-statement. Otherwise in cases where have not been detected the second `imshow()` will attempt to use `cimg` which will be empty leading to an error.

In the C++ equivalent of this example a check is made whether the vector (here in Python we have a list) actually has any circles in it that is whether the Hough circles function has managed to find any in the given image. This check is missing for the Python example and if no circles are found the application breaks.

Prevent grabcut, houghcircles and houghlines python2 demos from running automatically while browsing demos in the demo list viewer

houghlines.py for HoughLinesP() and HoughLines()
houghcircles.py for HoughCircles()