print error when deinterlacing fails

Originally committed as revision 15201 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -730,6 +730,7 @@ static void pre_process_video_frame(AVInputStream *ist, AVPicture *picture, void
             if(avpicture_deinterlace(picture2, picture,
                                      dec->pix_fmt, dec->width, dec->height) < 0) {
                 /* if error, do not deinterlace */
+                fprintf(stderr, "Deinterlacing failed\n");
                 av_free(buf);
                 buf = NULL;
                 picture2 = picture;