Documents freeze break

[Cosimo Cecchi <cosimoc gnome org>]

Hi,

I would like to get two last-minute fixes in the 3.4 release of
Documents.

Fix a crasher when clicking the 'Back' button from preview:
https://bugzilla.gnome.org/show_bug.cgi?id=672723

Fix loading spinner getting stuck in the way when clicking 'Back' and
the document preview is still loading:
https://bugzilla.gnome.org/show_bug.cgi?id=672724

Patches are attached to the bug and should both be trivial and very
safe.

Thanks for considering,
Cosimo