[llvm] [BOLT] Add 'constant island' check in scanExternalRefs to prevent a crash when the function is disassembled fail or skipped (PR #165577)

[Asher Dobrescu via llvm-commits]

https://github.com/Asher8118 commented:

I agree with Gergely, it would be nice to add a comment to document the new change.

Otherwise the patch looks good to me. I will leave this for one of the maintainers to give their approval to merge.

https://github.com/llvm/llvm-project/pull/165577