ostree-repo-pull: Fix a leak of the summary data if loading from cache

ostree-repo-pull: Fix a leak of the summary data if loading from cache

If the `summary_sig_not_modified` branch is taken above, both
`signatures` and `summary` are loaded from the cache. This makes the
`_ostree_repo_load_cache_summary_if_same_sig()` call below redundant (it
checks `signatures` matches the file it was just loaded from, and then
loads `summary` again) — but that call also currently overwrites
`summary` without clearing the old value.

Fix this by only making that call if `signatures` was retrieved, but the
server said the local `summary` cache was invalid.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>