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/x509.c b/library/x509.c
index 33dc0b9..4b34486 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -943,7 +943,7 @@
     const char *short_name = NULL;
     char s[MBEDTLS_X509_MAX_DN_NAME_SIZE], *p;
 
-    mbedtls_platform_memset( s, 0, sizeof( s ) );
+    memset( s, 0, sizeof( s ) );
 
     name = dn;
     p = buf;