Fix crash on prev/next change
Fix crash on prev/next change

When the currently displayed change was not arrived at from a
query listing (for example, was directly opened), there may be no
change list to find in the navigation history.  Don't try to use
it if that's the case.

Change-Id: I6fdb1ed2e16675bf1be3726b836130b695225b6b

aspiers/gertty,openstack/gertty,stackforge/gertty