Clean up call of writemap() in ac_encounter()

Pass the more obviously correct plane_owner instead of player->cnum.
They're the same, actually.

When deities could still fly foreign planes (before commit 2023b47c),
they weren't.  ac_encounter() updated the plane owner's in-memory
bmap, but wrote the current player's bmap to disk.

src/lib/subs/aircombat.c

@@ -226,7 +226,7 @@ ac_encounter(struct emp_qelem *bomb_list, struct emp_qelem *esc_list,
     }
     /* If the map changed, update it */
     if (changed)
-	writemap(player->cnum);
+	writemap(plane_owner);
 
     free_shiplist(&head);
     for (cn = 1; cn < MAXNOC; cn++)