quick fix, simplify cd callback behaviour

Signed-off-by: Ava Affine <ava@sunnypup.io>

src/stl/posix.rs

@@ -762,6 +762,10 @@ fn cd_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, Traceback> {
                         None,
                     ));
 
+                if !syms.contains_key(&CD_USER_CB.to_string()) {
+                    return Ok(Ctr::None)
+                }
+
                 match syms.call_symbol(
                     &CD_USER_CB.to_string(),
                     &Seg::new(),
@@ -990,18 +994,4 @@ pub fn load_posix_shell(syms: &mut SymTable, shell_state: Rc<RefCell<ShellState>
             ..Default::default()
         },
     );
-
-    syms.insert(
-        String::from(CD_USER_CB),
-        Symbol {
-            name: String::from(CD_USER_CB),
-            args: Args::None,
-            conditional_branches: false,
-            docs: String::from("lambda called at each invocation of cd"),
-            value: ValueType::Internal(Rc::new(move |_ast: &Seg, _sym: &mut SymTable| -> Result<Ctr, Traceback> {
-                Ok(Ctr::None)
-            })),
-            ..Default::default()
-        },
-    );
 }