bug#4016: Acknowledgement (23.1.50; docview: scrolling up breaks after setting a slice)

[Paer Kurlberg]

In case it might be helpful: below is a backtrace.  It seems that the
problem is that image-get-display-property (called in the function
image-next-line) returns the list

((slice 0 0 1100 1600) (image :type png :file
  "/tmp/docview1000/arnold.dvi-b04e9949bb98146d331479915a45fc5b/page-1.png"
  :pointer arrow))

and therefore image-size complains since a "proper image" should be a
list whose car equals 'image.


BACKTRACE: 

Debugger entered--Lisp error: (error "Invalid image specification")
  image-size(((slice 0 0 1100 1600) (image :type png :file
  "/tmp/docview1000/arnold.dvi-b04e9949bb98146d331479915a45fc5b/page-1.png"
  :pointer arrow)))
  image-next-line(36)
  image-scroll-up(nil)
  doc-view-scroll-up-or-next-page()
  call-interactively(doc-view-scroll-up-or-next-page nil nil)