res_rtp_asterisk: Clearing the remote RTCP address causes RTCP failures

When a call gets put on hold RTP is temporarily stopped and Asterisk was
setting the remote RTCP address to NULL. Then when RTCP data was received
from the remote endpoint, Asterisk would be missing this information when
publishing the rtcp_message stasis event. Consequently, message subscribers
(in this case res_hep_rtcp) trying to parse the "from" field output the
following error:

"ast_sockaddr_split_hostport: Port missing in (null)"

This patch makes it so the remote RTCP address is no longer set to NULL when
stopping RTP. There was only one place that appeared to check if the remote
RTCP address was NULL as a way to tell if RTCP was running. This patch added
an additional check on the RTCP schedid for that case to make sure RTCP was
truly not running.

ASTERISK-26860 #close

Change-Id: I6be200fb20db647e48b5138ea4b81dfa7962974b