#81 glePolyCylinder w/ loop causes segfault

[Mike C. Fletcher]

glePolyCylinder as used in the knots demo can crash if
the start-point of the polyline is included later in
the polyline. If it doesn't crash, it gives just weird
results (the caps appear to be generated with something
like a 0-div result as a multiplier, so they wind up
going off to infinity). This might be fixed by
upgrading the library (there's a new version included
with newer GLUT libs), or may possibly be a problem
with how we're passing the values (I don't think so,
though).