Possible race condition in dataReader's Read method

[kayrus]

Running an SMTP server with race detection (go run -race) showed a potential race condition in:

go-smtp/data.go

Line 67 in 30169ac

if r.limited {

and

go-smtp/data.go

Line 143 in 30169ac

if r.limited {

and

go-smtp/conn.go

Line 653 in 30169ac

r.limited = false

[foxcpp]

That seems to indicate that you have Read's on data reader running after Data completes which is not supported. I think we should update documentation to explicitly mention this.