I noticed several places in flatpak-dir.c that didn't check for
NULL progress, so lets move the check inside the implementation so
we can ensure its always checked.
To avoid the complexities of passing (and chaining) OstreeAsyncProgress
objects around, we only create one just before calling to ostree.
The rest of flatpak only ever uses the new FlatpakProgress object.
Co-authored by: Philip Chimento <philip@endlessm.com>