mgw: rx CRCX: Avoid sending dummy rtp if remote address not provided

The following sequence of events was seen frequently in a osmo-mgw
instance running on the field with heavy traffic:
"""
endpoint:rtpbridge/1@mgw CRCX: creating new connection ...
mgcp_network.c:236 endpoint:rtpbridge/1@mgw CI:1C8CCFA9 Failed to send dummy RTP packet.
"""

Allegedly, that happens because at CRCX time the remote address may
still not be known, hence we end up trying to send a dummy rtp packet
to, for instance, host 0.0.0.0 port 0, which will of course fail.
Let's avoid sending it if the address is not yet known.

Similary, same issue could be seen during MDCX, since at MDCX we don't
necessarily need to have a valid addr+port (there could be several MDCX
and only last one set it).

Finally, the keepalive timer also needs the check, since it iterates
over all connections, and it could be that some is still not fully
configured.

Related: SYS#5498
Change-Id: I8ceafda691146823b12232b4a804b4ce74acbdc8
3 files changed