Fix update to take dead units off carriers

upd_plane() upd_land() and left planes and land units lost to lack of
maintenance on their carriers.  Cargo lists were fine anyway, because
unit_cargo_init() ignored dead units.  But when the dead unit got
reused for building a new one, pln_prewrite() / lnd_prewrite() got
confused and attempted to take it off its carrier, which made
clink_rem() oops, because the unit wasn't on the cargo list.  No real
harm done, as oops recovery was fine.

Fix upd_plane() and upd_land() to clear the carrier.  Make
unit_cargo_init() oops when it finds dead units on carriers.

src/lib/common/cargo.c

@@ -241,24 +241,33 @@ unit_cargo_init(void)
 	unit_onresize(i);
 
     for (i = 0; (pp = getplanep(i)); i++) {
-	if (!pp->pln_own)
+	if (!pp->pln_own) {
+	    if (CANT_HAPPEN(pp->pln_ship >= 0 || pp->pln_land >= 0))
+		pp->pln_ship = pp->pln_land = -1;
 	    continue;
+	}
 	if (CANT_HAPPEN(pp->pln_ship >= 0 && pp->pln_land >= 0))
 	    pp->pln_land = -1;
 	pln_carrier_change(pp, EF_SHIP, -1, pp->pln_ship);
 	pln_carrier_change(pp, EF_LAND, -1, pp->pln_land);
     }
     for (i = 0; (lp = getlandp(i)); i++) {
-	if (!lp->lnd_own)
+	if (!lp->lnd_own) {
+	    if (CANT_HAPPEN(lp->lnd_ship >= 0 || lp->lnd_land >= 0))
+		lp->lnd_ship = lp->lnd_land = -1;
 	    continue;
+	}
 	if (CANT_HAPPEN(lp->lnd_ship >= 0 && lp->lnd_land >= 0))
 	    lp->lnd_land = -1;
 	lnd_carrier_change(lp, EF_SHIP, -1, lp->lnd_ship);
 	lnd_carrier_change(lp, EF_LAND, -1, lp->lnd_land);
     }
     for (i = 0; (np = getnukep(i)); i++) {
-	if (!np->nuk_own)
+	if (!np->nuk_own) {
+	    if (CANT_HAPPEN(np->nuk_plane >= 0))
+		np->nuk_plane = -1;
 	    continue;
+	}
 	nuk_carrier_change(np, EF_PLANE, -1, np->nuk_plane);
     }
 }

src/lib/update/land.c

@@ -125,6 +125,7 @@ upd_land(struct lndstr *lp, int etus,
 		makelost(EF_LAND, lp->lnd_own, lp->lnd_uid,
 			 lp->lnd_x, lp->lnd_y);
 		lp->lnd_own = 0;
+		lp->lnd_ship = lp->lnd_land = -1;
 		return;
 	    }
 	    wu(0, lp->lnd_own,

src/lib/update/plane.c

@@ -110,6 +110,7 @@ upd_plane(struct plnstr *pp, int etus,
 		makelost(EF_PLANE, pp->pln_own, pp->pln_uid,
 			 pp->pln_x, pp->pln_y);
 		pp->pln_own = 0;
+		pp->pln_ship = pp->pln_land = -1;
 		return;
 	    }
 	    wu(0, pp->pln_own,