Silence a bogus warning about uninitialized m_size

If crypt_block != NULL the entire object crypt_pfx should be
guaranteed to be initialized, including m_size, which will have been
initialized either in allocate_large(), either directly or via
allocate_trace().

storage/innobase/row/row0merge.cc

 /*****************************************************************************
 
 Copyright (c) 2005, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2014, 2017, MariaDB Corporation.
+Copyright (c) 2014, 2018, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -4596,6 +4596,8 @@ row_merge_build_indexes(
 		DBUG_RETURN(DB_OUT_OF_MEMORY);
 	}
 
+	TRASH(&crypt_pfx, sizeof crypt_pfx);
+
 	if (log_tmp_is_encrypted()) {
 		crypt_block = static_cast<row_merge_block_t*>(
 			alloc.allocate_large(3 * srv_sort_buf_size,