gio: Don't allow skipping past the end of GLocalFileInputStream

The overridden implementation of the skip method for
GLocalFileInputStream allows skipping past the end of the file which is
inconsistent with the documentation.  Prevent this by first seeking to
the end of the file and then seeking backwards from there as much as
is necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=711048