zend_ast: Clean up zend_ast_export_quoted_str()

[TimWolla]

Following #22350.

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@iliaal FYI: Pointers should be const where possible and variables should have as small scope as possible.

[iliaal]

Makes sense. FYI zend_ast_export_qstr() right above still has the separate size_t i; declaration if you want to fold the same change into this one.

[TimWolla]

FYI zend_ast_export_qstr() right above still has the separate size_t i; declaration if you want to fold the same change into this one.

Makes sense. I've also aligned the functions further by adding a local c to zend_ast_export_quoted_str() instead of keeping the explicit cast in the if().