res/res_rtp_asterisk: Fix crash in debug from RTCP reports without report block

When RTCP debugging was enabled, an RTCP report without a report block would
cause a crash. This was due to the verbose output not checking to see if the
report_block pointer was NULl before dereferencing it.

This patch adds the necessary check to prevent printing any verbose output
if the far side hasn't provided us the information they should have.

ASTERISK-24791 #close
Reported by: JoshE
Tested by: JoshE
........

Merged revisions 431879 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431880 65c4cc65-6c06-0410-ace0-fbb531ad65f3