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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@406721 f38db490-d61c-443f-a65b-d21fe96a405b