dir: Fix a minor leak of an OSTree checksum

The checksum here can leak if `flatpak_dir_remote_load_cached_summary()`
returns false at least once.

Spotted by asan while running gnome-software:
```
Direct leak of 2925 byte(s) in 45 object(s) allocated from:
    #0 0x7f44774ba6af in __interceptor_malloc (/lib64/libasan.so.8+0xba6af)
    flatpak#1 0x7f44764c941a in g_malloc ../../source/glib/glib/gmem.c:130
    flatpak#2 0x7f445bc860e7 in ostree_checksum_from_bytes src/libostree/ostree-core.c:1599
    flatpak#3 0x7f445bdbea82 in flatpak_dir_remote_fetch_indexed_summary /opt/gnome/source/flatpak/common/flatpak-dir.c:12563
    flatpak#4 0x7f445bd9932e in flatpak_remote_state_ensure_subsummary /opt/gnome/source/flatpak/common/flatpak-dir.c:577
    flatpak#5 0x7f445bdbfd42 in _flatpak_dir_get_remote_state /opt/gnome/source/flatpak/common/flatpak-dir.c:12872
    flatpak#6 0x7f445bdc006c in flatpak_dir_get_remote_state_optional /opt/gnome/source/flatpak/common/flatpak-dir.c:12953
    flatpak#7 0x7f445be07886 in flatpak_transaction_ensure_remote_state /opt/gnome/source/flatpak/common/flatpak-transaction.c:2057
    flatpak#8 0x7f445be095c7 in flatpak_transaction_add_ref /opt/gnome/source/flatpak/common/flatpak-transaction.c:2732
    flatpak#9 0x7f445be09c37 in flatpak_transaction_add_update /opt/gnome/source/flatpak/common/flatpak-transaction.c:2940
    flatpak#10 0x7f445bdd202c in flatpak_installation_list_installed_refs_for_update /opt/gnome/source/flatpak/common/flatpak-installation.c:1103
    flatpak#11 0x7f445bf07824 in gs_flatpak_add_updates ../../source/gnome-software/plugins/flatpak/gs-flatpak.c:2082
    flatpak#12 0x7f445bf2e2b9 in gs_plugin_add_updates ../../source/gnome-software/plugins/flatpak/gs-plugin-flatpak.c:484
    flatpak#13 0x7f44770533b2 in gs_plugin_loader_call_vfunc ../../source/gnome-software/lib/gs-plugin-loader.c:620
    flatpak#14 0x7f447705430f in gs_plugin_loader_run_results ../../source/gnome-software/lib/gs-plugin-loader.c:748
    flatpak#15 0x7f447706cb03 in gs_plugin_loader_process_thread_cb ../../source/gnome-software/lib/gs-plugin-loader.c:3110
    #16 0x7f44769967ed in g_task_thread_pool_thread ../../source/glib/gio/gtask.c:1531
    flatpak#17 0x7f447650e760 in g_thread_pool_thread_proxy ../../source/glib/glib/gthreadpool.c:350
    flatpak#18 0x7f447650dd02 in g_thread_proxy ../../source/glib/glib/gthread.c:831
```

Signed-off-by: Philip Withnall <[email protected]>

common/flatpak-dir.c

@@ -12565,6 +12565,8 @@ flatpak_dir_remote_fetch_indexed_summary (FlatpakDir   *self,
           if (flatpak_dir_remote_load_cached_summary (self, old_cache_name, old_checksum, ".sub", NULL,
                                                       &old_summary, NULL, cancellable, NULL))
             break;
+
+          g_clear_pointer (&old_checksum, g_free);
         }
 
       if (old_summary)