Use plain memset() for session ID and Hello.Random

Those are public values (transmitted in the clear over the wire).
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index ad04934..747b9f4 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -1215,12 +1215,12 @@
 
     p = buf + 6 + ciph_len;
     ssl->session_negotiate->id_len = sess_len;
-    mbedtls_platform_memset( ssl->session_negotiate->id, 0,
+    memset( ssl->session_negotiate->id, 0,
             sizeof( ssl->session_negotiate->id ) );
     memcpy( ssl->session_negotiate->id, p, ssl->session_negotiate->id_len );
 
     p += sess_len;
-    mbedtls_platform_memset( ssl->handshake->randbytes, 0, 64 );
+    memset( ssl->handshake->randbytes, 0, 64 );
     memcpy( ssl->handshake->randbytes + 32 - chal_len, p, chal_len );
 
     /*
@@ -1735,7 +1735,7 @@
     MBEDTLS_SSL_DEBUG_BUF( 3, "client hello, session id", buf + 35, sess_len );
 
     ssl->session_negotiate->id_len = sess_len;
-    mbedtls_platform_memset( ssl->session_negotiate->id, 0,
+    memset( ssl->session_negotiate->id, 0,
             sizeof( ssl->session_negotiate->id ) );
     memcpy( ssl->session_negotiate->id, buf + 35,
             ssl->session_negotiate->id_len );
@@ -2863,7 +2863,7 @@
         if( ssl->handshake->new_session_ticket != 0 )
         {
             ssl->session_negotiate->id_len = n = 0;
-            mbedtls_platform_memset( ssl->session_negotiate->id, 0, 32 );
+            memset( ssl->session_negotiate->id, 0, 32 );
         }
         else
 #endif /* MBEDTLS_SSL_SESSION_TICKETS */