The previous implementation missed a nil check which caused a panic
when the package of a type was nil.
Fixes: golang/go#39899
Change-Id: I2dfb50d6b79f52df367e093e5d857cd70b7cef27
Reviewed-on: https://go-review.googlesource.com/c/tools/+/240537
Run-TryBot: Josh Baum <joshbaum@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>