ostree_builtin_pull: consistently set free-function on refs_to_fetch
ostree_builtin_pull: consistently set free-function on refs_to_fetch

We are relying on the GPtrArray to free its contents, but we only
give it a free-function on one code path.

Found by valgrind memcheck.

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

Closes: #559
Approved by: cgwalters

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