go/types: check for non-negative index in tparamIndex

go/types: check for non-negative index in tparamIndex

There are code paths (particularly error formatting or tracing) that
call tparamIndex before the type parameter is bound. We cannot rely on
the index being non-negative.

Change-Id: Ibad91c691b4f319b0c7b465a750b679a8a7af6a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/364715
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>