crosshair: prevent crash from command usage in headless mode

commands/CmdCD.c

@@ -3819,7 +3819,8 @@ CmdCrosshair(
 	TxError("Usage: %s off|x y \n", cmd->tx_argv[0]);
 	return;
     }
-    DBWSetCrosshair(w, &pos);
+    if (w)
+	DBWSetCrosshair(w, &pos);
 }
 
 

dbwind/DBWtools.c

@@ -558,6 +558,9 @@ DBWSetCrosshair(window, pos)
     MagWindow *window;
     Point *pos;			/* New crosshair location in coords of rootDef. */
 {
+    assert(window);
+    assert(pos);
+
     bool needUpdate = FALSE;
 
     if (WINDOW_DEF(window) != curCrosshair.def) needUpdate = TRUE;