go/types: don't return an array type with invalid length
In preparation for porting CL 361412, fix a discrepancy in go/types,
where [-1]T is returned for an array type with invalid length.
Change-Id: Ia32f5b66c9c561ccf0c32af1922fc4690c66dbc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/362738
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>