avfilter/dnn_filter_common: fix memleak

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavfilter/dnn_filter_common.c

@@ -159,4 +159,10 @@ void ff_dnn_uninit(DnnContext *ctx)
     if (ctx->dnn_module) {
         (ctx->dnn_module->free_model)(&ctx->model);
     }
+    if (ctx->model_outputnames) {
+        for (int i = 0; i < ctx->nb_outputs; i++)
+            av_free(ctx->model_outputnames[i]);
+
+        av_freep(&ctx->model_outputnames);
+    }
 }