fix for wounded soldiers in base defense missions

in a base defense mission, _craft == 0
a wounded soldier's craft == 0

make sure _craft != 0 to avoid confusing the generator.
fix for base assault saves

we don't always save a _dest, so we can't always expect to be able to load one.
fix for duplicate items in bases

when we add them to the battlescape, we should delete them from the base.
that way, when we recover them, we don't make duplicates, and ammo usage gets
accounted for properly.