Disallow moving a container to itself since this would crash i3.

src/con.c

@@ -914,6 +914,11 @@ bool con_move_to_mark(Con *con, const char *mark) {
         target = TAILQ_FIRST(&(target->focus_head));
     }
 
+    if (con == target) {
+        DLOG("cannot move the container to itself, aborting.\n");
+        return false;
+    }
+
     return _con_move_to_con(con, target, false, true, false);
 }