pjsip show channelstats: Prevent possible segfault when faxing

pjsip show channelstats: Prevent possible segfault when faxing

Under rare circumstances, it's possible for the original audio
session in the active_media_state default_session to be corrupted
instead of removed when switching to the t38/image media session
during fax negotiation.  This can cause a segfault when a "pjsip
show channelstats" attempts to print that audio media session's
rtp statistics.  In these cases, the active_media_state
topology is correctly showing only a single t38/image stream
so we now check that there's an audio stream in the topology
before attempting to use the audio media session to get the rtp
statistics.

Resolves: #592