cmd/compile: don't crash while reporting invalid alias cycle

Add a missing nil check in the formatting code for expression
nodes. Matches the nil checks in the same code.

Fixes #48301.

Change-Id: Ia9bfd3535254a94996ee190b544d95e15433d252
Reviewed-on: https://go-review.googlesource.com/c/go/+/348740


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>