media formats: update res_format_attr_opus & silk

media formats: update res_format_attr_opus & silk

In r419044, we changed how formats were handled, but the return value
of the format_parse_sdp_fmtp functions in res_format_attr_opus and
res_format_attr_silk were not updated, causing calls to fail.  Ran
into this when getting codec_opus working with Asterisk 13.

Once the return value was corrected, we were crashing in opus_getjoint
because of NULL format attributes.  I've fixed this as well in this
patch.

Review: https://reviewboard.asterisk.org/r/4371/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@431267 65c4cc65-6c06-0410-ace0-fbb531ad65f3