go/types, types2: shorten object map assertion

It's a fairly well-known invariant that each object must exist in the
object map and cannot be nil. This change just shortens a check of
that invariant.

Change-Id: Id15c158c3a9ad91cdc230fb0b84eb69b2451cbdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/722061
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>