Fix one more case tha failed fuzz test on OSX.
Also removed some unsigned compares against zero that were causing warnings.
Another fuzz test fix: corruption check of the deep sample count table
was susceptible to 32-bit overflow.
Fix same integer overflow in deep tiled sample count corruption check.
Merge pull request #110 from openexr/osx_fuzzfix

Fix failed fuzz test cases on OSX.
Merge branch 'develop', remote branch 'upstream/develop' into develop