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/entropy.c b/library/entropy.c
index 8f28733..281ed23 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -370,7 +370,7 @@
     }
     while( ! done );
 
-    mbedtls_platform_memset( buf, 0, MBEDTLS_ENTROPY_BLOCK_SIZE );
+    memset( buf, 0, MBEDTLS_ENTROPY_BLOCK_SIZE );
 
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
     /*