[llvm] [BOLT][AArch64] Check Last Element Instead of Returning `nullptr` in `lookupStubFromGroup` (PR #114015)

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis commented:

Sorry, another nit I've missed.

Consider renaming your test file to something like `long-jmp-one-stub.s`

https://github.com/llvm/llvm-project/pull/114015