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>

libavcodec/libopenjpegenc.c

@@ -200,6 +200,9 @@ static opj_image_t *mj2_create_image(AVCodecContext *avctx, opj_cparameters_t *p
 
     img = opj_image_create(numcomps, cmptparm, color_space);
 
+    if (!img)
+        return NULL;
+
     // x0, y0 is the top left corner of the image
     // x1, y1 is the width, height of the reference grid
     img->x0 = 0;