Bugfix: check_availability() - negative missing days

[Magnus167]

Closes #913

[codecov]

Codecov Report

Patch coverage: 68.42% and no project coverage change.

Comparison is base (28b6345) 74.48% compared to head (ecd25c8) 74.49%.

❗ Current head ecd25c8 differs from pull request most recent head c3293c3. Consider uploading reports for the commit c3293c3 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #914   +/-   ##
========================================
  Coverage    74.48%   74.49%           
========================================
  Files           31       31           
  Lines         3833     3834    +1     
========================================
+ Hits          2855     2856    +1     
  Misses         978      978           

Files Changed	Coverage Δ
macrosynergy/management/check_availability.py	47.69% <68.42%> (+0.81%)	⬆️

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