assertion I crudely commented out has now been replaced with code
that clearly shows what you did wrong in the failing situation.
[originally from svn r8872]
long chain to a square numbered so low that the start of the chain
would have to go into negative numbers should not crash the game,
particularly when it happens as a momentary in-passing illustration.
I've fixed it for the moment just by removing the assertion. There's
probably a better fix which causes something less strange to happen
to the display as a result.
[originally from svn r8867]
