go/types, types2: don't crash when using trace with an invalid argument

go/types, types2: don't crash when using trace with an invalid argument

This only affects tests, typically manual tests, but when using trace
we're debugging and we don't want to crash because of trace itself.

No test because a test would cause trace output. Manually verified.

Fixes #60649.

Change-Id: I97abdb94db05774801ec5da56171f4a1aff35615
Reviewed-on: https://go-review.googlesource.com/c/go/+/501415
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>