libopenjpegenc: add NULL check for img before accessing it

If opj_image_create fails to allocate an image it returns NULL, which
causes a segmentation fault at 'img->x0 = 0'.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
