res_pjsip_sdp_rtp: Fix issue where sending a hold SDP twice could cause an unhold.

res_pjsip_sdp_rtp: Fix issue where sending a hold SDP twice could cause an unhold.

This change fixes a bug where if an SDP with media address and sendonly was
received twice the underlying call would go off hold, instead of remaining on hold.
This occured because the code did not properly take into account that the SDP
may contain both a valid media address and the sendonly attribute.

The code now examines the sendonly attribute and media address first, so if the
SDP is received again no change will occur.

ASTERISK-23558 #comment Reported by: John Bigelow

Review: https://reviewboard.asterisk.org/r/3472/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@413119 65c4cc65-6c06-0410-ace0-fbb531ad65f3