Fix CodeQL warning

[m-ringler]

Description

Check null before dereferencing.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Github actions

Checklist

• I followed the Contributing Guidelines.
• I did a self-review.
• I updated the version of czicompress
  • Bug fix -> PATCH
• New and existing unit tests pass locally with my changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.77%. Comparing base (35c5c16) to head (fdd82a8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
- Coverage   77.87%   77.77%   -0.11%     
==========================================
  Files          59       59              
  Lines        1921     1921              
  Branches      122      122              
==========================================
- Hits         1496     1494       -2     
- Misses        401      403       +2     
  Partials       24       24              

Components	Coverage Δ
czicompress	66.75% <ø> (ø)
czishrink	80.50% <ø> (-0.13%)	⬇️