avfilter/formats: add av_warn_unused_result to function prototypes

This uses the av_warn_unused_result attribute liberally to catch some forms of improper
usage of functions defined in avfilter/formats.h.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
