Bugfix IOStream backend

[JonasIsensee]

Fixes issues on the fallback IOStream backend where loading might fail with EOFError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.38%. Comparing base (e1585cd) to head (0de2c1f).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #713   +/-   ##
=======================================
  Coverage   87.37%   87.38%           
=======================================
  Files          37       37           
  Lines        4587     4588    +1     
=======================================
+ Hits         4008     4009    +1     
  Misses        579      579           

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