Cast uintptr difference to correct type. Fixes #704

[kamphaus]

Fixes #704

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This change is 

[codecov]

Codecov Report

Merging #705 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #705   +/-   ##
=======================================
  Coverage   81.94%   81.94%           
=======================================
  Files         164      164           
  Lines        8983     8983           
=======================================
  Hits         7361     7361           
  Misses       1220     1220           
  Partials      402      402

Impacted Files	Coverage Δ
util/goast.go	88.65% <100%> (ø)	⬆️

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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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