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>
diff --git a/src/mux_h2.c b/src/mux_h2.c
index fbc7963..85c8a3f 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3133,7 +3133,7 @@
 					TRACE_PROTO("failed to receive preface", H2_EV_RX_PREFACE|H2_EV_PROTO_ERR, h2c->conn);
 					h2c->st0 = H2_CS_ERROR2;
 					if (b_data(&h2c->dbuf) ||
-					    !(((const struct session *)h2c->conn->owner)->fe->options & PR_O_IGNORE_PRB))
+					    !(((const struct session *)h2c->conn->owner)->fe->options & (PR_O_NULLNOLOG|PR_O_IGNORE_PRB)))
 						sess_log(h2c->conn->owner);
 				}
 				goto fail;