net/smtp: SendMail & StartTLS need a non-nil tls.Config now

[bradfitz]

As pointed out by https://golang.org/cl/70380043/ , net/smtp's SendMail &
StartTLS need a non-nil tls.Config now that crypto/tls requires that.

Anywhere else in the standard library too?

[bradfitz]

Comment 1:

This issue was closed by revision a18bfb8.

Status changed to Fixed.