bicubic interpolation function do wrong indexing (reference negative index)

Question

I'm recently working on bicubic interpolation for college term-project.

I looked up some other codes in web and run it.

But 'every' codes has a same problem of indexing.

I thought there must be fault of me, but I can't find any.

inline unsigned char getpixel(unsigned char* in,
    int src_width, int src_height, int y, int x, int channel)
{
    if (x < src_width && y < src_height)
        return in[(y * 3 * src_width) + (3 * x) + channel];

    return 0;
}

void bicubicresize(unsigned char *in, unsigned char *out, int src_width, int src_height, int dest_width, int dest_height)
{

    const float tx = float(src_width) / dest_width;
    const float ty = float(src_height) / dest_height;
    const int channels = 3;
    const int row_stride = dest_width * channels;

    unsigned char C[5] = { 0 };

    for (int i = 0; i < dest_height; ++i)
    {
        for (int j = 0; j < dest_width; ++j)
        {
            const int x = int(tx * j);
            const int y = int(ty * i);
            const float dx = tx * j - x;
            const float dy = ty * i - y;

            for (int k = 0; k < 3; ++k)
            {
                for (int jj = 0; jj < 4; ++jj)
                {
                    const int z = y - 1 + jj;
                    unsigned char a0 = getpixel(in, src_width, src_height, z, x, k);
                    unsigned char d0 = getpixel(in, src_width, src_height, z, x - 1, k) - a0;
                    unsigned char d2 = getpixel(in, src_width, src_height, z, x + 1, k) - a0;
                    unsigned char d3 = getpixel(in, src_width, src_height, z, x + 2, k) - a0;
                    unsigned char a1 = -1.0 / 3 * d0 + d2 - 1.0 / 6 * d3;
                    unsigned char a2 = 1.0 / 2 * d0 + 1.0 / 2 * d2;
                    unsigned char a3 = -1.0 / 6 * d0 - 1.0 / 2 * d2 + 1.0 / 6 * d3;
                    C[jj] = a0 + a1 * dx + a2 * dx * dx + a3 * dx * dx * dx;

                    d0 = C[0] - C[1];
                    d2 = C[2] - C[1];
                    d3 = C[3] - C[1];
                    a0 = C[1];
                    a1 = -1.0 / 3 * d0 + d2 - 1.0 / 6 * d3;
                    a2 = 1.0 / 2 * d0 + 1.0 / 2 * d2;
                    a3 = -1.0 / 6 * d0 - 1.0 / 2 * d2 + 1.0 / 6 * d3;
                    out[i * row_stride + j * channels + k] = a0 + a1 * dy + a2 * dy * dy + a3 * dy * dy * dy;
                }
            }
        }
    }
}

If you look at the bicubicresize code carefully, in first loop, variable 'z' will be '-1'.

That will leads referencing negative index in image; out-of memory. and this show memory error in runtime.

I found other codes too, but they also use same kind of indexing.

What's wrong with me? or indexing? I'm so confused right now.

Help me please!