Mail: fixed reading with fully filled buffer (ticket #2159).

Mail: fixed reading with fully filled buffer (ticket #2159).

With SMTP pipelining, ngx_mail_read_command() can be called with s->buffer
without any space available, to parse additional commands received to the
buffer on previous calls.  Previously, this resulted in recv() being called
with zero length, resulting in zero being returned, which was interpreted
as a connection close by the client, so nginx silently closed connection.

Fix is to avoid calling c->recv() if there is no free space in the buffer,
but continue parsing of the already received commands.