Fix an issue in detection of the squares for the left diagonal

Summary: There's an off-by-one error here which incorrectly detects these
positions as the left diagonal:

   | |
  -+-+-
  X| |X
  -+-+-
   |X|

Adjust the calculation so the left diagonal is checked correctly.
Merge pull request #1 from epriestley/master

Fix an issue in detection of the squares for the left diagonal