Fix bogus message when deity attempts to navigate foreign ship

Fix bogus message when deity attempts to navigate foreign ship

Much of the code assumes that only the ship's owner can navigate it.
The assumption is correct, because shp_nav() leaves foreign ships
behind with a bogus "was sunk at" message (suppressed for country #0).

It would be nice to let deities navigate foreign ships, but the
assumption is not trivial to remove.  For now, just avoid the bogus
message.

Historical note: it looks like deities used to be able to navigate
foreign ships just fine until Empire 2 factored common code out of
navigate, sail and autonav.