adaptivedemux2: Call GTasks's return functions for blocking tasks

Gio/Task states the following:

If a GTask has been constructed and its callback set, it is an error to not call g_task_return_*() on it. GLib will warn at runtime if this happens (since 2.76).

Not quite sure why these were purposely left out for blocking requests? Running on GLib 2.78, Glib spit out a warning for each downloaded segment. Have not experienced any of them with this patch applied, also no other issues encountered so far.