Do not send RTP to port 0

Previously it was possible to send RTP data to port 0. This produce
multiple errors during the beginning of RTP transmission.

To address this OSMO_RTP_F_DISABLED flag was introduced. It's set by
default for all new RTP sessions. It can be manually unset after the
call to osmo_rtp_socket_create(). When the flag is set it prevents
transmission and reception of RTP frames for the session. The flag is
unset automatically in osmo_rtp_socket_connect() when session is bound
to non-zero remote port.

Fixes: OS#1662
diff --git a/include/osmocom/trau/osmo_ortp.h b/include/osmocom/trau/osmo_ortp.h
index c02cca8..9512759 100644
--- a/include/osmocom/trau/osmo_ortp.h
+++ b/include/osmocom/trau/osmo_ortp.h
@@ -29,6 +29,7 @@
 
 /*! \brief Flag to indicate the socket is in polling-only mode */
 #define OSMO_RTP_F_POLL		0x0001
+#define OSMO_RTP_F_DISABLED	0x0002
 
 /*! \brief A structure representing one RTP socket */
 struct osmo_rtp_socket {