main/rtp_engine: Fix crash when processing more than one RTCP report info block

Asterisk - in res_rtp_asterisk - only understands a single RTCP report info
block. When the RTCP information was refactored in the RTP Engine to be pushed
over the Stasis message bus, I put in the hooks into the engine to handle
multiple RTCP report info blocks, in the hope that a future RTP implementation
would be able to provide that data. Unfortunately, res_rtp_asterisk has a
tendency to "lie":
(1) It will send RTCP reports with a reception_report_count greater than 1
    (which is pulled directly from the RTCP packet itself, so that part is
    correct)
(2) It will only provide a single report block

When the rtp_engine goes to convert this to a JSON blob, hilarity ensues as it
looks for a report block that doesn't exist.

This patch updates the rtp_engine to be a bit more skeptical about what it is
presented with. While this could also be fixed in res_rtp_asterisk, this patch
prefers to fix it in the engine for two reasons:
(1) The engine is designed to work with multiple RTP implementation, and hence
    having it be more robust is a good thing (tm)
(2) res_rtp_asterisk's handling of RTCP information is "fun". It should report
    the correct reception_report_count; ideally it should also be giving us all
    of the blocks - but it is *definitely* not designed to do that. Going down
    that road is a non-trivial effort.

Review: https://reviewboard.asterisk.org/r/4158/

ASTERISK-24489 #close
Reported by: Gregory Malsack
Tested by: Gregory Malsack

ASTERISK-24498 #close
Reported by: Beppo Mazzucato
Tested by: Beppo Maazucato
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Merged revisions 427762 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 427763 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@427771 f38db490-d61c-443f-a65b-d21fe96a405b