appstream: Fix leak
appstream: Fix leak

Move some autoptr to the inner scope of the loop over each
appstream branch to avoid leaking.

Closes: #1964
Approved by: mwleeds

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak