rtp_engine: improved handling of get_rtp_info failure

rtp_engine: improved handling of get_rtp_info failure

In ast_rtp_instance_make_compatible(), after a failure of
channel tech call get_rtp_info() to return peer_instance,
the null pointer would be passed to ao2_ref, producing an
error that looked like a refernce counting problem but is
not.  This patch corrects that and adds helpful LOG_ERROR
messages to indicate which failure path occurred.

(issue AST-1276)
Review: https://reviewboard.asterisk.org/r/3156/
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Merged revisions 406721 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 406722 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@406723 65c4cc65-6c06-0410-ace0-fbb531ad65f3