Nil dereference panic fix

[artemseleznev]

The Svace static analyzer (https://www.ispras.ru/en/technologies/svace/) flagged this code as suspicious: it could lead to a panic (nil pointer dereference) because spec might be nil.
We have not spec != nil checks in all callers, so this check'd prevent panics

[fredbi]

ok thanks

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.04%. Comparing base (f09028f) to head (1644bd9).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
spec.go	0.00%	2 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   76.92%   76.04%   -0.88%     
==========================================
  Files           4        4              
  Lines         260      263       +3     
==========================================
  Hits          200      200              
- Misses         45       47       +2     
- Partials       15       16       +1     

Flag	Coverage Δ
oldstable-macos-latest	76.04% <0.00%> (-0.88%)	⬇️
oldstable-ubuntu-latest	76.04% <0.00%> (-0.88%)	⬇️
oldstable-windows-latest	76.04% <0.00%> (-0.88%)	⬇️
stable-macos-latest	76.04% <0.00%> (-0.88%)	⬇️
stable-ubuntu-latest	76.04% <0.00%> (-0.88%)	⬇️
stable-windows-latest	76.04% <0.00%> (-0.88%)	⬇️

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