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

[Rahman Lavaee via llvm-commits]

rlavaee wrote:

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

Thanks. Explained in the description.

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