Avoid possible crash when getting server properties.

[rtiemann]

The crash occurs when calling dls_device_get_all_props() for a content
directory server that we have not yet subscribed to (that is,
prv_cds_subscribed() returns FALSE in
prv_get_system_update_id_for_props()). This crash is caused by an
invalid GVariantBuilder passed to g_variant_builder_end() in
prv_get_sleeping_for_props(), leading to a NULL result pointer being
passed to dls_async_task_complete(). A GVariant is attempted to be
constructed from this NULL pointer in dls_task_complete().

Here is the call chain that leads to the crash:

dls_device_get_all_props()
prv_get_system_update_id_for_props()
gupnp_service_proxy_begin_action("GetSystemUpdateID") -> prv_system_update_id_for_props_cb()
prv_system_update_id_for_props_cb()
prv_get_sr_token_for_props()
prv_get_sleeping_for_props()
dls_async_task_complete()
cb_data->cb() -> prv_async_task_complete()
prv_async_task_complete()
dls_task_complete()
g_variant_new()

The crash was most likely observed when a device running Plex Media
Server was present on the network.

This commit moves the call of g_variant_builder_end() in
prv_system_update_id_for_props_cb() (which invalidates the
GVariantBuilder used later in prv_get_sleeping_for_props()) to the
error handling branch. This leaves the GVariantBuilder alone and
allows prv_get_sr_token_for_props() or one of its descendants to call
g_variant_builder_end() and complete the task.

[phako]

Applied to my fork at https://github.com/phako/dleyna-server/

[phako]

Included in release v0.7.0 at mentioned fork https://github.com/phako/dleyna-server/