GD2 stores the number of horizontal and vertical chunks as words (i.e. 2
byte unsigned). These values are multiplied and assigned to an int when
reading the image, what can cause integer overflows. We have to avoid
that, and also make sure that either chunk count is actually greater
than zero. If illegal chunk counts are detected, we bail out from
reading the image.

(cherry picked from commit 5b5d9db3988b829e0b121b74bb3947f01c2796a1)

We must not pretend that there are image data if there are none. Instead
we fail reading the image file gracefully.

(cherry picked from commit cdb648dc4115ce0722f3cc75e6a65115fc0e56ab)

For use with u2 values which don't deserve a custom macro...