Renamed hmac_ctx to opad and removed null check.

this array is now part of the struct and not dynamically allocated
so it can't be null.

include/psa/crypto_struct.h

@@ -88,7 +88,7 @@ typedef struct {
         /** The hash context. */
         struct psa_hash_operation_s hash_ctx;
         /** The HMAC part of the context. */
-        uint8_t hmac_ctx[PSA_CRYPTO_MD_BLOCK_SIZE];
+        uint8_t opad[PSA_CRYPTO_MD_BLOCK_SIZE];
 } psa_hmac_internal_data;