How to get a field value in form validate
Refering to form API Drupal 7 for #value

$form['field_a']['und']['#value']



  Used by: button, hidden, image_button, submit, token, value
  
  Description: Used to set values that cannot be edited by the user.


For $form_state['values']['field_a']['und'][0]['value']


  $form_state is one of the arguments passed to a form submission
  handler, or a form validation handler; its main usage is to retrieve
  the values entered from the user in the form see the content of
  $form_state['values'])


So you should use $form_state['values']['field_a']['und'][0]['value']