Fix undefined behavior leading to crash with clang

This code crashes with clang 3.4.1+, most likely due to arithmetics to null pointer, which is undefined behavior. I've fixed two apparent cases, there may be more.

PS. I'll be unavailable for 2 weeks, after that I'll try to find other cases.
PPS. FYI, there's bunch of other code fixes to catdoc in Debian, you'd probably like to review and merge them as well. Catdoc it still in high demand, and I really hope we can bring it back to shape.