Error highlighting bug in Bridges: when counting the number of extra

Error highlighting bug in Bridges: when counting the number of extra
bridges that could go to an island, the game was not correctly
accounting for existing bridges in all circumstances.

(E.g. in 7x7m2:2a6a2a1g4a6c4i1a1h23c2b, connect the top right 1 to
the 2 left of it, and the 6 left of that correctly lights up red.
But now connect the 6 to the 6 below it, and it wrongly unlights
again.)

[originally from svn r8905]