avoid using asserts

[stlankes]

for some reasons, the assert doesn't work in this context.
A workaround is using a simple if-statement instead.

[codecov]

Codecov Report

Merging #183 (ef037b2) into master (6ff81f4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #183   +/-   ##
=======================================
  Coverage   30.76%   30.76%           
=======================================
  Files          16       16           
  Lines        4017     4017           
=======================================
  Hits         1236     1236           
  Misses       2781     2781           

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[mkroening]

Ah, my bad!

I accidentally changed the behavior in fa62a6d#diff-9940bd2f66049a07423515adc7f6c054b3bb7bdd260b59b3b74e1f3007c0830cL680.

This change is semantic and has nothing to do with assert.

Instead, it should be:

assert!(!input, "Invalid I/O operation");