avfilter/buffersrc: add av_warn_unused_result attributes

This adds av_warn_unused_result whenever it is relevant.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavfilter/buffersrc.h

@@ -78,6 +78,7 @@ unsigned av_buffersrc_get_nb_failed_requests(AVFilterContext *buffer_src);
  * This function is equivalent to av_buffersrc_add_frame_flags() with the
  * AV_BUFFERSRC_FLAG_KEEP_REF flag.
  */
+av_warn_unused_result
 int av_buffersrc_write_frame(AVFilterContext *ctx, const AVFrame *frame);
 
 /**
@@ -98,6 +99,7 @@ int av_buffersrc_write_frame(AVFilterContext *ctx, const AVFrame *frame);
  * This function is equivalent to av_buffersrc_add_frame_flags() without the
  * AV_BUFFERSRC_FLAG_KEEP_REF flag.
  */
+av_warn_unused_result
 int av_buffersrc_add_frame(AVFilterContext *ctx, AVFrame *frame);
 
 /**
@@ -115,6 +117,7 @@ int av_buffersrc_add_frame(AVFilterContext *ctx, AVFrame *frame);
  * @return            >= 0 in case of success, a negative AVERROR code
  *                    in case of failure
  */
+av_warn_unused_result
 int av_buffersrc_add_frame_flags(AVFilterContext *buffer_src,
                                  AVFrame *frame, int flags);