src: Fix GVariant leaks
src: Fix GVariant leaks

g_task_propagate_pointer() gives ownership of the data to the caller, so
the GVariants handed this way must be unref'ed after the contents have
been extracted/copied.

BUG=https://github.com/ibus/ibus/pull/1956

Review URL: https://codereview.appspot.com/330710043

Patch from Carlos Garnacho <carlosg@gnome.org>.

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