Calling Clear() wrongly modified any other wxImage objects reusing the
same data.
Fix this by adding the missing call to AllocExclusive() to this function
and also add a check for the image validity which was missing there as
well ever since this function was added back in fc3762b5fc (add
wxImage::Clear (patch by troelsk); closes#10141, 2009-01-09).
Also add a unit test.
See #23553.
(cherry picked from commit 92664881fed59ffe3650886d6ea215e75d262981)
