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 openssl#21088

CLA: trivial

Reviewed-by: Tomas Mraz <[email protected]>
Reviewed-by: Bernd Edlinger <[email protected]>
Reviewed-by: Neil Horman <[email protected]>
(Merged from openssl#23898)

Author: adrien-zinger

Diff for: crypto/bn/rsaz_exp_x2.c

@@ -576,11 +576,7 @@ static void to_words52(BN_ULONG *out, int out_len,
         out_len--;
     }
 
-    while (out_len > 0) {
-        *out = 0;
-        out_len--;
-        out++;
-    }
+    memset(out, 0, out_len * sizeof(BN_ULONG));
 }
 
 static ossl_inline void put_digit(uint8_t *out, int out_len, uint64_t digit)