Use plain memset() for freshly allocated objects

This commits reverts to plain memset() for cases like:

    some_type foo;
    memset( &foo, 0, sizeof( foo ) );

(Sometimes there is code between declaration in memset(), but it doesn't
matter as long as it doesn't touch foo.)

The reasoning is the same as in the previous commit: the stack shouldn't
contain sensitive data as we carefully wipe it after use.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index ddc58a4..e9102a7 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3376,7 +3376,7 @@
             const size_t max_len = rec->data_len + padlen;
             const size_t min_len = ( max_len > 256 ) ? max_len - 256 : 0;
 
-            mbedtls_platform_memset( tmp, 0, sizeof( tmp ) );
+            memset( tmp, 0, sizeof( tmp ) );
 
             switch( mbedtls_md_get_type(
                         mbedtls_md_get_handle( &transform->md_ctx_dec ) ) )