OpenGL ES - glReadPixels

I am taking a screenshot with glReadPixels to perform a "cross-over" effect between two images.

On the Marmalade SDK simulator, the screenshot is taken just fine and the "cross-over" effect works a treat:

However, this is how it looks on iOS and Android devices - corrupted:
_{(source: eikona.info)}

I always read the screen as RGBA 1 byte/channel, as the documentation says it's ALWAYS accepted.

Here is the code used to take the screenshot:

uint8* Gfx::ScreenshotBuffer(int& deviceWidth, int& deviceHeight, int& dataLength) {

    /// width/height
    deviceWidth = IwGxGetDeviceWidth();
    deviceHeight = IwGxGetDeviceHeight();
    int rowLength = deviceWidth * 4; /// data always returned by GL as RGBA, 1 byte/each

    dataLength = rowLength * deviceHeight;

    // set the target framebuffer to read
    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    uint8* buffer = new uint8[dataLength];
    glReadPixels(0, 0, deviceWidth, deviceHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    return buffer;
}

void Gfx::ScreenshotImage(CIwImage* img, uint8*& pbuffer) {

    int deviceWidth, deviceHeight, dataLength;

    pbuffer = ScreenshotBuffer(deviceWidth, deviceHeight, dataLength);
    img->SetFormat(CIwImage::ABGR_8888);
    img->SetWidth(deviceWidth);
    img->SetHeight(deviceHeight);
    img->SetBuffers(pbuffer, dataLength, 0, 0);
}

In the end, it was lack of memory. The "new uint8[dataLength];" never returned an existent pointer, thus the whole process went corrupted.

TomA, your idea of clearing the buffer actually helped me to solve the problem. Thanks.