af_compand: make sure request_frame always outputs at least one frame

This fixes a segmentation fault because request_frame in fifo.c assumes
that the call to ff_request_frame will populate fifo->root.next.
Before, it was possible for request_frame in af_compand to not do this,
resulting in a null pointer access. Now, request_frame in af_compand
always will return at least one frame or an error, as per the API
specifications in avfilter.h for request_frame.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit d3cfd7aff86ee3d449ca68aba21d67b9b2136a9b)
Signed-off-by: Anton Khirnov <anton@khirnov.net>