[libunwind] [libunwind] Fix wrong end argument passed to decodeEHHdr() (PR #68813)

[Alexander Richardson via cfe-commits]

arichardson wrote:

> > Can we merge this into #68815 please?
> 
> I'd like to keep the two commits separate since they are addressing different bugs.

This would be a lot easier if I could chose rebase+merge in the PR flow.

https://github.com/llvm/llvm-project/pull/68813