Fix warnings around mixing int and size_t usage in unsafe ways

[acoates-ms]

Summary

react-native-windows builds treat a lot of c++ compiler warnings as errors. Including ones that cause value truncation when assigning size_t values to an int.

This fixes the compiler warning, by using size_t as the type for i, which matches size. I then modified the loop to avoid the value underflow that occurs when decrementing an unsigned zero value.

Changelog

[Internal] [Changed] - Fix compiler warnings around mixing int and size_t usage in unsafe ways

Test Plan

This code change was made in react-native-windows when integrating latest react-native changes. -- our fabric implementation was able to run through this code without any issues.

[acoates-ms]

This PR merged in this fix into react-native-windows: microsoft/react-native-windows#10688

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	7,741,366	+54
android	hermes	armeabi-v7a	7,143,654	-9
android	hermes	x86	8,051,895	-85
android	hermes	x86_64	8,023,615	+67
android	jsc	arm64-v8a	9,608,659	+67
android	jsc	armeabi-v7a	8,373,912	-1
android	jsc	x86	9,555,978	-89
android	jsc	x86_64	10,148,477	+64

Base commit: abb21dd
Branch: main

[facebook-github-bot]

@rozele has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: abb21dd
Branch: main

[react-native-bot]

This pull request was successfully merged by @acoates-ms in 138c88c.

^{When will my fix make it into a release? | Upcoming Releases}

[NickGerleman]

I think you could pattern match off of the -Wpedantic change to add -Wextra and prevent as much error level mismatch in the future.