In gimp_data_factory_finalize(), wait on the factory's async set
after canceling it, and before continuing destruction.  It's not
generally safe to just abandon an async op without waiting on it
-- this is a font-specific hack, due to the fact we can't actually
cancel font loading, and GimpFontFactory is prepared to handle
this.

Instead, in gimp_font_factory_finalize(), cancel and clear the
async set, so that GimpDataFactory doesn't actually wait for
loading to finish.

In gimp_font_factory_load_async_callback(), don't try to acess the
factory when the operation is canceled, since cancelation means the
factory is already dead.  On the other hand, when the opeation
isn't canceled, make sure to thaw the container even when font
loading failed, so that we always match the freeze at the begining
of the operation.