Only multiply the values of either the (left,right) or (bottom,top) dimension.
When you take the factor (screen_size.x / screen_size.y) for example it should be like this if I'm not mistaken:

camera->left = -cam_boarder * (screen_size.x / screen_size.y);
camera->right = cam_boarder * (screen_size.x / screen_size.y);
camera->bottom = -cam_boarder;
camera->top = cam_boarder;