fortify: Do not special-case 0-sized destinations

All fake flexible arrays should have been removed now, so remove the
special casing that was avoiding checking them. If a destination claims
to be 0 sized, believe it. This is especially important for cases where
__counted_by is in use and may have a 0 element count.

Link: https://lore.kernel.org/r/20240619203105.work.747-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>