[llvm-branch-commits] [YAML] Don't validate `Fill::Size` after error (PR #123280)

[Vitaly Buka via llvm-branch-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/123280

Size is required, so we don't know if it's in
uninitialized state after the previous error.

Triggers msan on llvm/test/tools/yaml2obj/ELF/custom-fill.yaml