Revert "opentxs: fix -Wtautological-undefined-compare"

This reverts commit 101c9652be73b48fa1f0213470a7fb95c8c3365e.
opentxs: re-fix -Wtautological-undefined-compare
Merge pull request #148 from toxeus/fixWtautological

opentxs: re-fix -Wtautological-undefined-compare