errors: omit redundant nil check in type assertion for Join
When ok is true, err can't be nil.
Make it behave more like the Unwrap function.
Change-Id: Ieba5de57d60f5ff4d6a3468d703e6f72be02a97d
GitHub-Last-Rev:
6df9365a1d586bba64df57be11f9ca7b772cc773
GitHub-Pull-Request: golang/go#74764
Reviewed-on: https://go-review.googlesource.com/c/go/+/689920
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>