res_rtp_asterisk: Add frame list cleanups to ast_rtp_read

In Asterisk 16+, there are a few places in ast_rtp_read where we've
allocated a frame list but return a null frame instead of the list.
In these cases, any frames left in the list won't be freed.  In the
vast majority of the cases, the list is empty when we return so
there's nothing to free but there have been leaks reported in the
wild that can be traced back to frames left in the list before
returning.

The escape paths now all have logic to free frames left in the
list.

ASTERISK-28609
Reported by: Ted G

Change-Id: Ia1d7075857ebd26b47183c44b1aebb0d8f985f7a