Port to platforms where NULL is a pointer

* src/image.c (x_create_x_image_and_pixmap):
The Picture type is an integer, not a pointer.
Come to think of it, Picture is really XID, and as
far as I can tell, all-bits-one is the invalid XID,
not all-bits-zero.  However that is a bigger issue;
for now, this patch just unbreaks the build.
1 job for master in 47 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #589