(nuk_postread): Update nuke location to match that of the plane

(nuk_postread): Update nuke location to match that of the plane
carrying it.  This is required since arming doesn't remove nukes from
the nuke file anymore (arm.c rev. 1.15).

(nuk_prewrite): Properly destroy nuke.  This is required since nuke
file contains individual nukes instead of stockpiles (nuke.h
rev. 1.19).