Ensure edid_cksum is never NULL (coredump fix)

Signed-off-by: Alexey Galakhov <agalakhov@gmail.com>
Revert "Fix coredump on NULL EDID"

This reverts commit 700b8b19a4918d8ab4128035498015644485af2b.
Now edid_cksum is never NULL.

Signed-off-by: Alexey Galakhov <agalakhov@gmail.com>
Check XAtom data validity

Signed-off-by: Alexey Galakhov <agalakhov@gmail.com>