C++: Use unsigned size type for loop index

[johnbeard]

This avoids a wrong sign conversion when the loop expression casts to a negative signed int.

Fixes: https://github.com/kaitai-io/kaitai_struct/issues/1220

---

I did intend to add a test for this based on the reproducing case in the linked issue, but I could not figure out how to actually run the tests.

I tried build-tests cpp_stl in the tests repo, but it errored out with complaints about Julia(?):

[error] /home/john/src/kaitai_struct/tests/translator/src/main/scala/io/kaitai/struct/testtranslator/specgenerators/JuliaSG.scala:6:8: object JuliaCompiler is not a member of package io.kaitai.struct.languages
[error] did you mean JavaCompiler or LuaCompiler?
[error] import _root_.io.kaitai.struct.languages.JuliaCompiler

How can I build/run the tests? I'll be more than happy to add a test if I know how!

[johnbeard]

I figured out adding a test: https://github.com/kaitai-io/kaitai_struct_tests/pull/133