iter: propagate panics from the iterator passed to Pull
This change propagates panics from the iterator passed to Pull through
next and stop. Once the panic occurs, next and stop become no-ops (the
iterator is invalidated).
For #67712.
Change-Id: I05e45601d4d10acdf51b53e3164bd891c1b324ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/589136 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>