[release-branch.go1.17] go/types: fix panic with nil package name
This is a straightforward port of CL 320490 to go/types.
Fixes #48082
Change-Id: I763c806c777f926a563d8f9384764e5b3f7f083c
Reviewed-on: https://go-review.googlesource.com/c/go/+/324732
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
(cherry picked from commit
e32fab145b488cf8bd1fb4a2466cb9be33ac3770)
Reviewed-on: https://go-review.googlesource.com/c/go/+/347071