bn2binpad: Use memset as the buffer will be used later
bn2binpad: Use memset as the buffer will be used later

Apparently using OPENSSL_cleanse() confuses the fuzzer so it
makes the buffer to appear uninitialized. And memset can be
safely used here and it is also potentially faster.

Fixes #17237

Reviewed-by: Bernd Edlinger <ac6902843ae77cc0d49b9c8ddec4cbf141db1bb7@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/17240)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl