Error trying to encode an empty image with JPEG encoder (Bug #2604)

Hi,

I was applying some unitary tests to some personal code when I discovered that OpenCV throws an error when you try to encode an empty image with imencode. I suppose that it would be the normal behaviour, but maybe a more descriptive error message could be useful for developers. I checked that in OpenCV 2.4.3 code, the JPEG encoder doesn't check if you are dealing or not with and empty image.

A simple snippet of code that reproduces this error is:

#include <vector>
#include <opencv2/highgui/highgui.hpp>

int main()
{
  cv::Mat img;
  std::vector<uchar> jpegImg;
  cv::imencode(".jpg", img, jpegImg);
}

And the error throw by opencv is this one:

OpenCV Error: Assertion failed (code) in imencode, file /.../OpenCV-2.4.3/modules/highgui/src/loadsave.cpp, line 429
terminate called after throwing an instance of 'cv::Exception'
  what():  /.../OpenCV-2.4.3/modules/highgui/src/loadsave.cpp:429: error: (-215) code in function imencode

Do you consider this behaviour a bug? Does a change of this behaviour worth it? I would like to contribute in the project, and I could start with something simple like this ^^