Fix crashes in res_pjsip_sdp_rtp and res_pjsip_t38 when a stream is rejected and...

Fix crashes in res_pjsip_sdp_rtp and res_pjsip_t38 when a stream is rejected and external_media_address is set.

The callback function for changing the media address in streams wrongly assumes that a connection line
will always be present. This is false as no line is present if a stream has been rejected.

(closes issue ASTERISK-22645)
Reported by: Rusty Newton
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Merged revisions 400360 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400361 65c4cc65-6c06-0410-ace0-fbb531ad65f3