AudioBook: Fix crash when opening an audiobook (bgo#727623)

The exception was a GLib.MissingIntPtrCtorException.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>
Fixup: Fix crash when opening Metadata Fixer (bgo#728381)

The exception was a GLib.MissingIntPtrCtorException.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>