go/types: fix assertion failure when range over int is not permitted

go/types: fix assertion failure when range over int is not permitted

Fixes an assertion failure in Checker.rangeStmt that range over int
only has a key type and no value type. When allowVersion failed,
rangeKeyVal returns Typ[Invalid] for the value instead of nil. When
Config.Error != nil, rangeStmt proceeded. The check for rhs[1]==nil
was not enough to catch this case. It must also check rhs[1]==

Updates #68334

Change-Id: Iffa1b2f7b6a94570ec50b8c6603e727a45ba3357
Reviewed-on: https://go-review.googlesource.com/c/go/+/597356
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>