Fix for requesting a high-score that is out of range; fixes off-by-one on highscores.

There were two problems with high-scores. First, highscore_value() and highscore_name() are both 1-indexed. Second, if a high-score is requested but not available, the program should return 0 for the value and "Unknown" for the name (rather than crashing).

This commit fixes both of these issues.