add nil-check for decoded private key pem bytes

[rockspore]

close #359

[codecov]

Codecov Report

Merging #360 (3ba0377) into 2.0.x (1e7bb20) will increase coverage by 1%.
The diff coverage is 100%.

@@         Coverage Diff          @@
##           2.0.x   #360   +/-   ##
====================================
+ Coverage     88%    89%   +1%     
====================================
  Files          9      9           
  Lines        760    762    +2     
====================================
+ Hits         667    674    +7     
+ Misses        49     46    -3     
+ Partials      44     42    -2     

Impacted Files	Coverage Δ
server/authmanager.go	69% <100%> (+7%)	⬆️

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