Revert change to return behaviour in psa_reserve_free_key_slot

This change was a mistake, we still need to wipe the pointers here.

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index 3accacf..8d7ff90 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -199,7 +199,7 @@
         status = psa_key_slot_state_transition(selected_slot, PSA_SLOT_EMPTY,
                                                PSA_SLOT_FILLING);
         if (status != PSA_SUCCESS) {
-            return status;
+            goto error;
         }
 
         *volatile_key_id = PSA_KEY_ID_VOLATILE_MIN +