cmd/compile/internal/types2: add a check for nil reason in assignableTo

cmd/compile/internal/types2: add a check for nil reason in assignableTo

A recent change to error message formatting was missing a nil check.

Fixes #49592

Change-Id: Ic1843e0277ba75eec0e8e41fe34b59c323d7ea31
Reviewed-on: https://go-review.googlesource.com/c/go/+/364034
Trust: Robert Findley <rfindley@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>