shell: Refactor to avoid lint complaining about dropping reference

The lint is right in that dropping the reference does little, but it
ensures that the reference is not held longer than the lock. Using an
explicit scope achieves the same without any discussions about the drop.

src/shell.rs

@@ -108,11 +108,13 @@ pub unsafe trait CommandListInternals: Sized {
         let sleeve = lock
             .as_ref()
             .expect("Callback called while no shell set up as running");
-        // unsafe: A suitable callback is always configured. We can make a &mut out of it for as
-        // long as we hold the lock.
-        let root = unsafe { &mut *(sleeve.0 as *mut Self) };
-        let result = root.find_self_and_run(argc, argv, command_index);
-        drop(root);
+        let result;
+        {
+            // unsafe: A suitable callback is always configured. We can make a &mut out of it for as
+            // long as we hold the lock.
+            let root = unsafe { &mut *(sleeve.0 as *mut Self) };
+            result = root.find_self_and_run(argc, argv, command_index);
+        }
         drop(lock);
         result
     }