[PATCH] Fix undefined behavior in AArch64

[David Majnemer]

>> The current tests trigger this behavior.
> 
> Thanks for spotting this. Could I ask what platform this is on, out of
> curiosity?

I compiled llvm+clang with some -fsanitize

> 
> The patch looks good regardless; there's not much we can do about UB
> other than make tests fail in at least one case.
> 
> Would you like me to commit it, or are you able?

Committed in r179317

> 
> Tim.