Fix NULL-pointer issue with RTP stats.

When we originate a call from an extension to another we
get a NULL pointer issue when going on hook.

Enable CONFIG_PACKAGE_asterisk-res-clioriginate and then
use the command below:

asterisk -rx 'channel originate local/#123457@local_extensions
extension 0000@local_extensions'

After going on hook on the FXS port, which is mapped to extension
0000, there will be a segmentation fault because the other channel
doesn't implement the getRtpStats() function pointer.