Make empty key slots explicit

Add new status field to key slots, and use it.

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 3b8a319..36d5410 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -221,6 +221,7 @@
     if (status != PSA_SUCCESS) {
         goto exit;
     }
+    slot->status = PSA_SLOT_OCCUPIED;
 
 #if defined(MBEDTLS_PSA_CRYPTO_SE_C)
     /* Special handling is required for loading keys associated with a
@@ -315,6 +316,7 @@
     /* Copy actual key length and core attributes into the slot on success */
     slot->key.bytes = key_buffer_length;
     slot->attr = attributes.core;
+    slot->status = PSA_SLOT_OCCUPIED;
 
 exit:
     if (status != PSA_SUCCESS) {