'Fix' an assertion failure during play: accidentally connecting a

'Fix' an assertion failure during play: accidentally connecting a
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]