Fix GCC compilation -Waggressive-loop-optimizations

Fix GCC compilation -Waggressive-loop-optimizations

GCC 13.1.0 were reporting a compilation warning with -O2/3 and
-Waggressive-loop-optimizations. GCC is raising an undefined behavior in the
while loop. Replace the while loop with a memset call at the top of the
function.

Fixes #21088

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23898)