avfilter/internal: add av_warn_unused_result

av_warn_unused_result is added to functions whose return status should
be checked. Currently does not trigger any warnings, but should be
useful for future robustness.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
