Fix a panic in suproc rule when the declaration of the variable is not available in the AST

[ccojocar]

fixes #726

[codecov-commenter]

Codecov Report

Merging #728 (30e321b) into master (ff17c30) will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #728      +/-   ##
==========================================
- Coverage   69.60%   69.56%   -0.04%     
==========================================
  Files          47       47              
  Lines        2859     2862       +3     
==========================================
+ Hits         1990     1991       +1     
- Misses        813      814       +1     
- Partials       56       57       +1     

Impacted Files	Coverage Δ
rules/subproc.go	86.88% <33.33%> (-2.77%)	⬇️

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