If XGetImage fails to create image, don't dereference it to bounds check

Reported by gcc 7.3:

GetImage.c:110:25: warning: potential null pointer dereference [-Wnull-dereference]
  if (planes < 1 || image->height < 1 || image->bytes_per_line < 1 ||
                    ~~~~~^~~~~~~~

Introduced by 8ea762f9 in Xlib 1.6.4

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>