Introduce helper macro for presence of stream ciphersuites

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 10ddf0d..f3cf8f5 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -683,7 +683,7 @@
     /*
      * Encrypt
      */
-#if defined(MBEDTLS_CIPHER_NULL_CIPHER)
+#if defined(MBEDTLS_SSL_SOME_MODES_USE_STREAM)
     if( mode == MBEDTLS_MODE_STREAM )
     {
         int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
@@ -708,7 +708,7 @@
         }
     }
     else
-#endif /* MBEDTLS_CIPHER_NULL_CIPHER */
+#endif /* MBEDTLS_SSL_SOME_MODES_USE_STREAM */
 
 #if defined(MBEDTLS_GCM_C) || \
     defined(MBEDTLS_CCM_C) || \
@@ -1245,7 +1245,7 @@
     }
 #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
 
-#if defined(MBEDTLS_CIPHER_NULL_CIPHER)
+#if defined(MBEDTLS_SSL_SOME_MODES_USE_STREAM)
     if( mode == MBEDTLS_MODE_STREAM )
     {
         padlen = 0;
@@ -1266,7 +1266,7 @@
         }
     }
     else
-#endif /* MBEDTLS_CIPHER_NULL_CIPHER */
+#endif /* MBEDTLS_SSL_SOME_MODES_USE_STREAM */
 #if defined(MBEDTLS_GCM_C) || \
     defined(MBEDTLS_CCM_C) || \
     defined(MBEDTLS_CHACHAPOLY_C)