ICU-22940 MF2 ICU4C: Error checking improvements in parser

[catamorphism]

Improve checking for OOM errors when allocating UnicodeSets, per post-merge comments on #3236

Checklist

• Required: Issue filed: ICU-22940
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[FrankYFTang]

Thanks for working on this.

[FrankYFTang]

why not just

UnicodeSet* isAlpha = unisets::gUnicodeSets[unisets::ALPHA] =  initAlpha(status);
if (U_FAILURE(status)) {
        return nullptr;
}

[catamorphism]

Fixed in e2990d2

[FrankYFTang]

I think you can simplified to

UnicodeSet* nameStart = unisets::gUnicodeSets[unisets::NAME_START] = initNameStartChars(status);
UnicodeSet* digit = unisets::gUnicodeSets[unisets:: DIGIT] = initDigits(status);
if (U_FAILURE(status)) {
   return nullptr;
}

[catamorphism]

Fixed in e2990d2

[FrankYFTang]

I think you can simplfied to

    UnicodeSet* content = unisets::gUnicodeSets[unisets::CONTENT] = initContentChars(status);
    UnicodeSet* whitespace = unisets::gUnicodeSets[unisets::WHITESPACE] = initWhitespace(status);

[catamorphism]

Fixed in e2990d2