Assert that key_insert() always inserts a new key.

src/keys.c

@@ -394,7 +394,7 @@ key_insert(VRT_CTX, uint8_t *id, uint8_t idlen, const uint8_t *key,
 	memcpy(k->key, key, AES128_KEYLEN);
 	memcpy(k->id, id, idlen);
 	k->idlen = idlen;
-	VRBT_INSERT(key_tree, tree_h, k);
+	AZ(VRBT_INSERT(key_tree, tree_h, k));
 	return (k);
 }