fire: Plug memory leak when missile defense sinks firing ships

fire: Plug memory leak when missile defense sinks firing ships

do_defdam() iterates over the list firing ships, applies return fire
to each, and frees its list element.  When it finds a ship that has
been sunk already, it skips it.  This also skips the free, leaking the
list element.  The leak goes back to flawed bug fix in Empire 2.

As far as I can see, missile defense is the only way a ship can be
sunk there.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>