Pointer arithmetic for negative array index. Fixes #708

[kamphaus]

Fixes #708

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This change is 

[codecov]

Codecov Report

Merging #709 into master will decrease coverage by 0.33%.
The diff coverage is 53.5%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage      82%   81.66%   -0.34%     
==========================================
  Files         164      165       +1     
  Lines        9042     9164     +122     
==========================================
+ Hits         7415     7484      +69     
- Misses       1223     1270      +47     
- Partials      404      410       +6

Impacted Files	Coverage Δ
transpiler/variables.go	83.92% <100%> (+0.38%)	⬆️
transpiler/unary.go	71.45% <43.18%> (-3.11%)	⬇️
util/evaluate.go	55.55% <55.55%> (ø)
noarch/string.go	100% <0%> (ø)	⬆️
program/function_definition.go	100% <0%> (ø)	⬆️
ast/weak_attr.go	55.55% <0%> (+2.61%)	⬆️

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[elliotchance]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[elliotchance]

Reviewed 1 of 2 files at r2, 2 of 2 files at r3.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable