Change behavior of psa_get_key_lifetime()

psa_get_key_lifetime() behavior changed regarding empty slots, now
it return the lifetime of and empty slots. Documentation in header
file updated accordingly.
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index ac763f9..07c1da1 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -103,10 +103,6 @@
  * Applications may call this function more than once. Once a call
  * succeeds, subsequent calls are guaranteed to succeed.
  * 
- * \note Initial lifetime value for each key slot is initiated 
- *       to PSA_KEY_LIFETIME_VOLATILE, user should change this value
- *       before calling psa_import_key() if needed.
- *
  * \retval PSA_SUCCESS
  * \retval PSA_ERROR_INSUFFICIENT_MEMORY
  * \retval PSA_ERROR_COMMUNICATION_FAILURE
@@ -609,9 +605,7 @@
  *         Success.
  * \retval PSA_ERROR_INVALID_ARGUMENT
  *         The key slot is invalid,
- *         or the key data is not correctly formatted.
- * \retval PSA_ERROR_EMPTY_SLOT
- *         The key slot is not occupied.         
+ *         or the key data is not correctly formatted.   
  */
 psa_status_t psa_get_key_lifetime(psa_key_slot_t key,
                                   psa_key_lifetime_t *lifetime);
@@ -630,11 +624,9 @@
  * \retval PSA_ERROR_INVALID_ARGUMENT
  *         The key slot is invalid,
  *         or the key data is not correctly formatted.
- * \retval PSA_ERROR_EMPTY_SLOT
- *         The key slot is not occupied.
  */
 psa_status_t psa_set_key_lifetime(psa_key_slot_t key,
-                                  const psa_key_lifetime_t lifetime);
+                                  psa_key_lifetime_t lifetime);
 
 /**@}*/
 
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 152fb17..fef0539 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1274,9 +1274,6 @@
         return( PSA_ERROR_INVALID_ARGUMENT );
 
     slot = &global_data.key_slots[key];
-
-    if( slot->type == PSA_KEY_TYPE_NONE )
-        return( PSA_ERROR_EMPTY_SLOT );
     
     *lifetime = slot->lifetime;