reflect: avoid a bounds check in stack-constrained code
Since CL 682496 we need more stack space to handle bounds checks.
The code modified here normally has no bounds checks, but in -N
builds it still does and thus uses too much stack.
Use unsafe arithmetic to avoid the bounds check.
This will hopefully fix some of the arm64 linux builders.
Change-Id: I5b3096a14b4fb9553e635b7f340e60b8ffba8755
Reviewed-on: https://go-review.googlesource.com/c/go/+/690415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@google.com>