fixed out-of-bounds bug when score was not added to high score list

fixed out-of-bounds bug when score was not added to high score list

If the score is not added to the local high score list for a solved
level (because the high score table is completely full, and the new
score is not better than the lowest score entry in the list), there
was an out-of-bounds bug caused (by accessing the score array at
position "-1", which was not correctly handled as "no score entry").

This uncaught error condition could have caused various misbehavior
(inclusing crashes) of the game. This change fixes this bug.