NULL-ptr crash in image_read_ahead_done_cb()

NULL-ptr crash in image_read_ahead_done_cb()

It's not easy to reproduce, but if one is mad and follows the steps and
presses the keys a few dozens times, there's a slight chance one can
trigger it. Not much to investigate it further.

Forwarding from Sami Farin 2015-08-07 16:40:44 EDT:

 | to reproduce: quickly keep pressing f (fullscreen) and page down
 | (next file) keys.
 |
 | attached patch is a kind of workaround, I did not inspect further
 | is this threading issue etc

Full backtrace: https://bugzilla.redhat.com/attachment.cgi?id=1027647

Patch: https://bugzilla.redhat.com/attachment.cgi?id=1060482&action=diff&context=patch&collapsed=&headers=1&format=raw