Fix type instability

[davidanthoff]

No description provided.

[codecov-io]

Codecov Report

Merging #66 into master will increase coverage by 1.58%.
The diff coverage is 88.67%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   79.31%   80.89%   +1.58%     
==========================================
  Files           2        2              
  Lines         116      157      +41     
==========================================
+ Hits           92      127      +35     
- Misses         24       30       +6

Impacted Files	Coverage Δ
src/C_interface.jl	85% <ø> (+0.78%)	⬆️
src/ReadStat.jl	80.29% <88.67%> (+1.94%)	⬆️

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