fire launch: Avoid "may be used uninitialized" warnings

fire launch: Avoid "may be used uninitialized" warnings

launch_missile() initializes @n to the target ship number only when
the target is a ship.  It also uses it only then, to re-get the ship.
Clang is cool with this, but GCC gets confused and warns "‘n’ may be
used uninitialized".

Use target_ship.shp_uid instead of @n to avoid the warning.

Similar issue in c_fire(): use vship.shp_uid instead of for @vhsipno.

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