load_metadata_internal: don't leak GBytes
load_metadata_internal: don't leak GBytes

Found by valgrind memcheck. g_variant_new_from_bytes takes a ref to the
bytes, so we need to release the original ref.

Signed-off-by: Simon McVittie <smcv@debian.org>

Closes: #556
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree