[lld] [lld-macho] Fix crash: ObjC category merge + relative method lists (PR #104081)

[via llvm-commits]

alx32 wrote:

> Can you add `-objc_relative_method_lists` to [lld/test/MachO/objc-category-merging-complete-test.s](https://github.com/llvm/llvm-project/commit/d82b6832201c4a30#diff-c4526cb248e1204d8f7adf96ea7694320e0f6ea880cda3c65b8d579fb50b6cf9) as well?

So looks like doing this exposes a separate issue with relative method lists in that test. I'll merge this as-is and address the other issue separately. 

https://github.com/llvm/llvm-project/pull/104081