[llvm] [Object][Wasm] Guard relocation diagnostics against out-of-range indexes (PR #204872)

[Sam Clegg via llvm-commits]

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sbc100 wrote:

Would it make more sense to test this at the lower level in `llvm/test/Object/Wasm/bad-relocation.yaml`?

https://github.com/llvm/llvm-project/pull/204872