Fixed crash on tapping on the last pixel on bottom

It's needed because Month grid has 7x6 tiles. But it is possible that only 7x5 is visible at once.
But as the height of month grid is `1.f + kTileSize.height * numWeeks` than it is possible to tap
on the last pixel on bottom and `hitTest:withEvent:` will return tile from invisible tile rows
and it will be without date.

`+1` the height calculation is needed for adding nice gray line at the bottom. It was added here:
https://github.com/klazuka/Kal/commit/f42d043fb3d2a34c5913aea9b18e41cfb463ef32#L1R131