Fix panic from not checking for nil

[jamesgoodhouse]

This addresses the long outstanding issue reported in #5.

[codecov]

Codecov Report

Merging #19 into master will decrease coverage by 0.13%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   48.96%   48.83%   -0.14%     
==========================================
  Files           4        4              
  Lines         339      342       +3     
==========================================
+ Hits          166      167       +1     
- Misses        164      165       +1     
- Partials        9       10       +1

Impacted Files	Coverage Δ
session/session.go	90.9% <50%> (-9.1%)	⬇️

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