Handle case where an unknown format is used to get the preferred codec size

In ast_codec_pref_getsize, if an unknown format is passed to the method,
no preferred codec will be selected and a negative number will be used to
index into the format list.  The method now logs an unknown format as a
warning, and returns an empty format list.

(issue ASTERISK-19655)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/1863/
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Merged revisions 362377 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@362380 65c4cc65-6c06-0410-ace0-fbb531ad65f3