bug fix: no longer segfaults when detecting surfaces near image boundaries

This bug was due to failing to check the boundaries of the image when looking up voxel properties. It only occurred when the detected objects were close to the image boundaries. This bug has been fixed.

NOTE: After submitting this release, I noticed that I forgot to also check that the detected voxel coordinates lie within the image mask (if applicable). I'll submit another commit after this to address this issue with masks. (Because its a smaller issue that does not effect as many users, I might not bother to create a new release.)