opentxs: fix -Wtautological-undefined-compare

Found by clang++ 3.5:
"Reference cannot be bound to dereferenced null pointer in
well-defined C++ code; comparison may be assumed to always
evaluate to true/false"
OTTransaction: fix -Wabsolute-value

Found by clang++ 3.5
Merge pull request #145 from toxeus/fixTautological

clang++ 3.5: fix compile errors