Fix panic on WeakAttr. Fixes #610

[elliotchance]

This change is 

[codecov]

Codecov Report

Merging #712 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #712      +/-   ##
==========================================
- Coverage   81.95%   81.92%   -0.03%     
==========================================
  Files         164      164              
  Lines        9016     9156     +140     
==========================================
+ Hits         7389     7501     +112     
- Misses       1223     1239      +16     
- Partials      404      416      +12

Impacted Files	Coverage Δ
ast/weak_attr.go	55.55% <100%> (+2.61%)	⬆️
types/cast.go	68.09% <0%> (+0.56%)	⬆️
transpiler/binary.go	74.21% <0%> (+4.17%)	⬆️

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[elliotchance]

Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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