BUG: Fixed failure in StataReader when reading variable labels in 117 files

Stata's implementation does not match the online dta file format description.
The solution used here is to directly compute the offset rather than reading
it from the dta file.  If Stata fixes their implementation, the original code
can be restored.
closes #7816
Merge pull request #7818 from bashtage/stata-117-variable-labels

BUG: Fixed failure in StataReader when reading variable labels in 117