BUG/MINOR: stream-int: don't try to read again when CF_READ_DONTWAIT is set

BUG/MINOR: stream-int: don't try to read again when CF_READ_DONTWAIT is set

Commit 9aaf778 ("MAJOR: connection : Split struct connection into struct
connection and struct conn_stream.") had to change the way the stream
interface deals with incoming data to accomodate the mux. A break
statement got lost during a change, leading to the receive call being
performed twice even when CF_READ_DONTWAIT is set. The most noticeable
effect is that it made the bug described in commit 33982cb ("BUG/MAJOR:
stream: ensure analysers are always called upon close") much easier to
reproduce as it would appear even with an HTTP frontend.

Let's just restore the stream-interface flag and the break here, as in
the previous code.

No backport is needed as this was introduced during 1.8-dev.