RTP_Emulation: Avoid failure if T_transmit triggers between bind and connect

During MGCP_Test's f_flow_modify, an RTP socket may be Tx-enabled, and
f_flow_modify first calls bind, then connect, with MDCX transaction in
the middle (which can take some time).

If T_transmit from RTP_Emulation triggers (RTP packet to be send),
during that time, TTCN3 will fail to send the packet:
RTP_Emulation.ttcn:312 Message enqueued on RTP from system @Socket_API_Definitions.PortEvent : { result := { errorCode := ERROR_SOCKET (4), connId := 2, os_error_code := 89, os_error_text := "Destination address required" } } id 1

Change-Id: I20e7aed35bb28200e30ee5efc718f77e036d8262
1 file changed