Before emptying the target_table lists, and then removing their
infrastructure, add some tolerance to an empty list.

Instead of bugging-out on an empty list, pretend we already
reached the end in the two-list-walk.
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200622113317.20477-3-james.morse@arm.com