vulkan: workaround llvmpipe atan2 zero case

[nihui]

No description provided.

[tencent-adm]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.96%. Comparing base (0f5c6ef) to head (611eb64).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #6729    +/-   ##
========================================
  Coverage   95.95%   95.96%            
========================================
  Files         970      970            
  Lines      403476   403142   -334     
========================================
- Hits       387159   386870   -289     
+ Misses      16317    16272    -45     

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