std: fix various nilness findings
Found by running
$ go run golang.org/x/tools/go/analysis/passes/nilness/cmd/nilness@latest std
No actual bugs--other than one panic(nil)--but a
few places where error nilness was unclear.
Change-Id: Ia916ba30f46f29c1bcf928cc62280169b922463a
Reviewed-on: https://go-review.googlesource.com/c/go/+/486675
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Alan Donovan <adonovan@google.com>