Corrected bug #1437.

I corrected bug #1437. Two signed-short values were not extracted corrected from an lParam coming from mouse events. The values had to be casted to signed-short before assigning to an int.
See recent discussion here http://code.opencv.org/issues/1437

This is my first pull request so let me know if I did anything wrong. I couldn't run the official opencv test set as the instructions seen here (http://code.opencv.org/projects/opencv/wiki/How_to_contribute#The-instruction-in-brief) aren't crystal clear. I clone opencv_extra but I don't know what I'm supposed to do with it...
I did create a small console app to validate my correction though.