Check if descriptor is nil when filtering unstable packages

[bufdev]

Fixes #211

[codecov]

Codecov Report

Merging #213 (7c0a30a) into master (1791ed5) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   68.98%   69.00%   +0.02%     
==========================================
  Files         232      232              
  Lines       12607    12609       +2     
==========================================
+ Hits         8697     8701       +4     
+ Misses       2931     2930       -1     
+ Partials      979      978       -1     

Impacted Files	Coverage Δ
internal/buf/bufcheck/internal/runner.go	78.26% <100.00%> (+0.64%)	⬆️
internal/buf/buffetch/internal/ref_parser.go	92.23% <0.00%> (+0.91%)	⬆️

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