libavfilter/vf_dnn_detect: Set used pointer to NULL

Set used pointer to NULL in case it leaks the storage.
Signed-off-by: Wenbin Chen <wenbin.chen@intel.com>

libavfilter/vf_dnn_detect.c

@@ -223,6 +223,7 @@ static int dnn_detect_parse_yolo_output(AVFrame *frame, DNNData *output, int out
                     av_freep(&bbox);
                     return AVERROR(ENOMEM);
                 }
+                bbox = NULL;
             }
     }
     return 0;