acme/autocert: fix context usage

Context.Err() is not valid before Context.Done().

Updates golang/go#19856

Change-Id: I7605bb227bfc4cb542ef3db49870d4928ce704d1
Reviewed-on: https://go-review.googlesource.com/40396
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Vaghin <ddos@google.com>
1 file changed