Add check for nil otherTypeDef

[farexw]

Relation issue
e.g. https://github.com/swaggo/swag/pull/118/files

This PR solve the problem

[sdghchj]

Sorry, I introduced this bug
pkgDefs.uniqueDefinitions[fullName] needs staying nil as a mark that the same fullName exists, and will be removed finally.

[farexw]

ok

[codecov]

Codecov Report

Base: 95.70% // Head: 95.43% // Decreases project coverage by -0.27% ⚠️

Coverage data is based on head (6e0677a) compared to base (38b5f15).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1372      +/-   ##
==========================================
- Coverage   95.70%   95.43%   -0.28%     
==========================================
  Files          14       14              
  Lines        2818     2824       +6     
==========================================
- Hits         2697     2695       -2     
- Misses         67       71       +4     
- Partials       54       58       +4     

Impacted Files	Coverage Δ
packages.go	88.98% <0.00%> (-3.20%)	⬇️

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[sdghchj]

LGTM.
Thank you!