go/types: guard against check==nil in newNamed
go/types: guard against check==nil in newNamed

When importing generic named types, it is possible for Checker.newNamed
to be called during type instantiation when the Checker is nil.

In this case we should be able to safely skip this delayed expansion.

Updates #45580

Change-Id: I75422100464d57eba24642c93e06e8b47d904fc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/322974
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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