pdf: Do not use EvDocument API in get_info implementation

get_info might be called while the document is being loaded and the
EvDocument cache is not filled yet. Use only the poppler API to make
sure we get valid values. This fixes the page size always reported as
0x0 in the properties dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=711093
xps: Do not use EvDocument API in get_info implementation

get_info might be called while the document is being loaded and the
EvDocument cache is not filled yet. Use only the GXPS API to make
sure we get valid values. This fixes the page size always reported as
0x0 in the properties dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=711093
djvu: remove un-needed assignment
shell: fix a possible null pointer dereference