[llvm] [llvm-dwarfdump-fuzzer] fix out of bounds potential (PR #76408)

[via llvm-commits]

DavidKorczynski wrote:

@nikic could you help review this PR as well? It solves a simiar problem as in https://github.com/llvm/llvm-project/pull/73888

https://github.com/llvm/llvm-project/pull/76408