在on_image_render函数下增加对输入图像rgb进行灰度化，结果会导致手机上闪退

[meixiansen]

具体代码
`void MyNdkCamera::on_image_render(cv::Mat& rgb) const
{
if (rgb.empty())
return;
cv::Mat src = rgb.clone();
cv::Mat matGray;
cv::cvtColor(src, matGray, CV_RGB2GRAY);
// nanodet
{
ncnn::MutexLockGuard g(lock);

    if (g_yolox)
    {
        std::vector<Object> objects;
        g_yolox->detect(rgb, objects);

        g_yolox->draw(rgb, objects);
    }
    else
    {
        draw_unsupported(rgb);
    }
}

draw_fps(rgb);

}进行debug，具体报错E/cv::error(): OpenCV(4.5.3) Error: Requested object was not found (could not open directory: /data/app/com.tencent.ncnnyolox-tvY0PkV6XrQyBx4J6SbZxw==/base.apk!/lib/arm64-v8a) in glob_rec, file /home/runner/work/opencv-mobile/opencv-mobile/opencv-4.5.3/modules/core/src/glob.cpp, line 273
A/libc: Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 in tid 975 (ImageReader-640), pid 32479 (ncent.ncnnyolox)`