Fixes issue with using -1 as size_t
Fixes issue with using -1 as size_t

clang on macOS didn't catch this!

HazyResearch/deepdive,HazyResearch/deepdive,HazyResearch/deepdive,HazyResearch/sampler,HazyResearch/sampler,HazyResearch/deepdive,HazyResearch/sampler