* avoid usage of next/last buffer after flush

(but there still something wrong when the first frame isn't keyframe)

Originally committed as revision 1403 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -677,7 +677,8 @@ void avcodec_flush_buffers(AVCodecContext *avctx)
            if(s->picture[i].data[0] && (   s->picture[i].type == FF_BUFFER_TYPE_INTERNAL
                                         || s->picture[i].type == FF_BUFFER_TYPE_USER))
             avctx->release_buffer(avctx, (AVFrame*)&s->picture[i]);
-        }
+	}
+	s->last_picture.data[0] = s->next_picture.data[0] = NULL;
         break;
     default:
         //FIXME