Fix copy-and-paste error in TermVectorOffsetInfo::setStartOffset()

The argument should be named startOffset, not endOffset, otherwise the
function is a no-op.
Fix some cases of adding integers to string literals.

Use StringUtils::toString() before trying to concatenate or use operator<<
which is more type-safe.
Merge pull request #43 from upthere/clang-static-analysis

Fix some issues identified using clang's static analysis