See the issue report for a description of the bug. The root cause is
dereferencing of NULL pointers, specifically the priv->document member of
EvWindow structures when no document is loaded. This commit adds checks
for a NULL value of priv->document at all the points in the file
shell/ev-window.c where this pointer was previously dereferenced without
being checked.

Fixes 357 (https://github.com/mate-desktop/atril/issues/357)

Update "Documented by" in About Atril dialog

    When annotating or filling a form in a document, this must be saved
    as a different document as atril does not overwrite documents.
    The user can expect to store the modified file in the same place
    than the original document, except when the document lives in
    a temporary directory (e.g. downloaded automatically with a web
    browser), in whose case it must fallback to the Documents
    directory (if set) or the the home directory.

    Previously, atril assumed the latest directory used, or the place
    where an image or attachment was stored last. Such behaviour is
    confusing because the latest place opened might have no relation
    with the document modified.

    Fixes https://bugzilla.gnome.org/show_bug.cgi?id=767611
    Adapted from https://gitlab.gnome.org/GNOME/evince/commit/319a6d49