Fix premature free'ing of the frame committed in r349608

Even though we set the frame to the ast_null_frame and return that,
the caller of the frame hook may still need the frame.  This now is
a bit more careful about when it frees the frame, i.e., only under
the same conditions that applied when we duplicated it in the first
place.
........

Merged revisions 349822 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@349823 65c4cc65-6c06-0410-ace0-fbb531ad65f3