TIFF: correct for invalid tile heights
TIFF: actually return the tile height

Caused by 64f33a89e3277a5c9a6d1bb6cbcd546e6f3a9b95.

This may be the most stupid bug that I have ever introduced.
Merge pull request #445 from melissalinkert/tiff-tile-height

TIFF: correct for invalid tile heights