Removed further timing differences during SSL message decryption in ssl_decrypt_buf()

New padding checking is unbiased on correct or incorrect padding and
has no branch prediction timing differences.

The additional MAC checks further straighten out the timing differences.
diff --git a/include/polarssl/sha2.h b/include/polarssl/sha2.h
index 054988d..39d9347 100644
--- a/include/polarssl/sha2.h
+++ b/include/polarssl/sha2.h
@@ -160,6 +160,9 @@
  */
 int sha2_self_test( int verbose );
 
+/* Internal use */
+void sha2_process( sha2_context *ctx, const unsigned char data[64] );
+
 #ifdef __cplusplus
 }
 #endif