Problems creating first person controller

Hi! I'm new here and in Unity. I was creating a first person controller script with this code:

using UnityEngine; using System.Collections;

public class CharacterController : MonoBehaviour {

 public float speed = 10f;

 private float moveHorizontal;
 private float moveVertical;
 private Vector3 movement;

 void Start () {
      Cursor.lockState = CursorLockMode.Locked;
 }    

 void Update () {
     moveHorizontal = Input.GetAxisRaw ("Horizontal");
     moveVertical = Input.GetAxisRaw ("Vertical");

     movement = new Vector3 (moveHorizontal, 0f, moveVertical);
     movement.Normalize ();

     transform.Translate (movement * Time.deltaTime * speed);

     if (Input.GetKeyDown (KeyCode.Escape))
         Cursor.lockState = CursorLockMode.None;
 }

}

The problem is when I use it in my scene because when the character (a capsule) bumps into a corner, it starts to rotate indefinitely.

This image tries to explain it: