Add missing zeroization of buffered handshake messages

This commit ensures that buffers holding fragmented or
future handshake messages get zeroized before they are
freed when the respective handshake message is no longer
needed. Previously, the handshake message content would
leak on the heap.

library/ssl_tls.c

@@ -8741,6 +8741,7 @@ static void ssl_buffering_free_slot( mbedtls_ssl_context *ssl,
     if( hs_buf->is_valid == 1 )
     {
         hs->buffering.total_bytes_buffered -= hs_buf->data_len;
+        mbedtls_platform_zeroize( hs_buf->data, hs_buf->data_len );
         mbedtls_free( hs_buf->data );
         memset( hs_buf, 0, sizeof( mbedtls_ssl_hs_buffer ) );
     }