[llvm] [NFC,SHT_LLVM_BB_ADDR_MAP] Fix ub in ELF.cpp. (PR #115830)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Sorry, is there a typo in the PR title (ub -> up?) or is there some undefined behaviour that this is fixing (if so, what)?

https://github.com/llvm/llvm-project/pull/115830