Fixed some NULL checks in EAP memory

  * Add a NULL check after the first talloc call
  * Fix the check after allocating eap_ds->request

Furthermore, I guess the code in eap_ds_free could be simplified, the talloc library should be able to free the response and request automatically.
Merge pull request #864 from qnet-herwin/talloc_null_check_eap_mem

Fixed some NULL checks in EAP memory