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_crt.c b/library/x509_crt.c
index 5919303..b7d1036 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -2271,7 +2271,7 @@
     p = buf;
     n = size;
 
-    mbedtls_platform_memset( &sig_info, 0, sizeof( mbedtls_x509_crt_sig_info ) );
+    memset( &sig_info, 0, sizeof( mbedtls_x509_crt_sig_info ) );
     mbedtls_pk_init( &pk );
 
     if( NULL == crt )