acme/autocert: validate SNI value even more

Follow-up to https://golang.org/cl/42497

Change-Id: I638e7ba5e924a74ce5318e4b50fb18082fd1a43f
Reviewed-on: https://go-review.googlesource.com/42499
Reviewed-by: Alex Vaghin <ddos@google.com>
2 files changed