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try this, it has a simpler way to do what your code does. there is still a dot on result image. i think you should use findContours() to get rid of it.

#include<opencv2/core.hpp>
#include<opencv2/imgproc.hpp>
#include<opencv2/highgui.hpp>

using namespace cv;

int main() {
    Mat srcImg = imread("test.png", IMREAD_GRAYSCALE);
    Mat dstImg = srcImg > 127;

    rectangle(dstImg, Rect(0, 0, dstImg.cols, dstImg.rows), Scalar(255));
    floodFill(dstImg, Point(0,0), Scalar(0));

    imshow("srcImg", srcImg);
    imshow("dstImg", dstImg);
    waitKey();

    return 0;
}

try this, it has a simpler way to do what your code does. but there is still a dot on result image. i think image when using floodFill() method.

so you should use findContours() to get rid of it.

#include<opencv2/core.hpp>
#include<opencv2/imgproc.hpp>
#include<opencv2/highgui.hpp>

using namespace cv;
using namespace std;

int main() {
    Mat srcImg = imread("test.png", IMREAD_GRAYSCALE);
    Mat dstImg = srcImg > 127;

    rectangle(dstImg, Rect(0, 0, dstImg.cols, dstImg.rows), Scalar(255));
    floodFill(dstImg, Point(0,0), Point(0, 0), Scalar(0));

    imshow("srcImg", srcImg);
    imshow("dstImg", dstImg);

    srcImg = srcImg > 127;
    vector<vector<Point> > contours;
    findContours(srcImg, contours, RETR_LIST, CHAIN_APPROX_SIMPLE);

    for (size_t i = 0; i < contours.size(); i++)
    {
        Rect bounding_rect = boundingRect(contours[i]);
        Rect test_rect = bounding_rect & Rect(1, 1, dstImg.cols - 2, dstImg.rows - 2);
        if (bounding_rect != test_rect)
        {
            drawContours(srcImg, contours, (int)i, Scalar(0),-1);
        }
    }
    imshow("Result using findContours", srcImg);
    waitKey();
     return 0;
}