Don't let POGO (#0) navigate and march ghosts

POGO can navigate dead ships, and march dead land units.  The ghosts
even get sighted and interdicted, and can hit mines (landmines only
until commit fe372539, v4.3.27).  Noted for ships in commit 9100af0b.
Has always been broken.  Fix by making shp_sel() and lnd_sel()
explicitly reject ghosts.

Same code pattern also exists in pln_sel, but dead plains fail the
efficiency test, so it's harmless there.  Apply the same fix anyway.

src/lib/subs/lndsub.c

@@ -412,7 +412,7 @@ lnd_sel(struct nstr_item *ni, struct emp_qelem *list)
 	 * but much of the code assumes that only the land unit's
 	 * owner can march it.
 	 */
-	if (land.lnd_own != player->cnum)
+	if (!land.lnd_own || land.lnd_own != player->cnum)
 	    continue;
 	if (opt_MARKET) {
 	    if (ontradingblock(EF_LAND, &land)) {

src/lib/subs/plnsub.c

@@ -538,7 +538,7 @@ pln_sel(struct nstr_item *ni, struct emp_qelem *list, struct sctstr *ap,
 	 * much of the code assumes that only the plane's owner can
 	 * fly it.
 	 */
-	if (plane.pln_own != player->cnum)
+	if (!plane.pln_own || plane.pln_own != player->cnum)
 	    continue;
 	if (plane.pln_mobil <= 0)
 	    continue;

src/lib/subs/shpsub.c

@@ -70,7 +70,7 @@ shp_sel(struct nstr_item *ni, struct emp_qelem *list)
 	 * much of the code assumes that only the ship's owner can
 	 * navigate it.
 	 */
-	if (ship.shp_own != player->cnum)
+	if (!ship.shp_own || ship.shp_own != player->cnum)
 	    continue;
 	mcp = &mchr[(int)ship.shp_type];
 	if (opt_MARKET) {