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 <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/17240)

(cherry picked from commit 858d5ac16d)

crypto/bn/bn_lib.c

@@ -505,7 +505,8 @@ int bn2binpad(const BIGNUM *a, unsigned char *to, int tolen, endianess_t endiane
     /* Swipe through whole available data and don't give away padded zero. */
     atop = a->dmax * BN_BYTES;
     if (atop == 0) {
-        OPENSSL_cleanse(to, tolen);
+        if (tolen != 0)
+            memset(to, '\0', tolen);
         return tolen;
     }