Fix g_assertion failure in bus_ibus_impl_{load,save}_global_engine_name functions.
Fix g_assertion failure in bus_ibus_impl_{load,save}_global_engine_name functions.

Add NULL checks to the functions so they don't abort with assertion failure even if the functions are called before the ibus configuration daemon gets ready.
See http://code.google.com/p/chromium-os/issues/detail?id=6689#c27 for details.

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