BUG/MINOR: mux-h2: Obey dontlognull option during the preface

BUG/MINOR: mux-h2: Obey dontlognull option during the preface

If a connection is closed during the preface while no data are received, if
the dontlognull option is set, no log message must be emitted. However, this
will still be handled as a protocol error. Only the log is omitted.

This patch should fix the issue #1336 for H2 sessions. It must be backported
to 2.4 and 2.3 at least, and probably as far as 2.0.

(cherry picked from commit 3f35da296ee30bba3783f343a586091c66f21c65)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 79b347d50f13d3a918871f82035ed7de8f08dd50)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>