runtime: fix iterator returns map entries after clear (pre-swissmap)

runtime: fix iterator returns map entries after clear (pre-swissmap)

Fixes #70189
Fixes #59411

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-noswissmap
Change-Id: I4ef7ecd7e996330189309cb2a658cf34bf9e1119
Reviewed-on: https://go-review.googlesource.com/c/go/+/625275
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>