Guard against the panic in golang/go#49397, though it should not be
possible with well-formed AST.
Updates golang/go#49397
Change-Id: I3428504a4cfd361c1f51316d8c9aeee4aa9d7f42
Reviewed-on: https://go-review.googlesource.com/c/tools/+/364675
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>