Prevent panic: unexpected constant value: <nil>

[ldemailly]

Fixes #1229

(or to be more correct... prevents it yet it's suspicious, see comments in the issue)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.49%. Comparing base (a836898) to head (8438d9f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
analyzers/conversion_overflow.go	0.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1232      +/-   ##
==========================================
- Coverage   68.53%   68.49%   -0.04%     
==========================================
  Files          75       75              
  Lines        4382     4384       +2     
==========================================
  Hits         3003     3003              
- Misses       1232     1233       +1     
- Partials      147      148       +1     

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