[BN] harden `BN_copy()` against leaks from memory accesses

`BN_copy()` (and indirectly `BN_dup()`) do not propagate the
`BN_FLG_CONSTTIME` flag: the propagation has been turned on and off a
few times in the past years, because in some conditions it has shown
unintended consequences in some code paths.

Without turning the propagation on once more, we can still improve
`BN_copy()` by avoiding to leak `src->top` in case `src` is flagged with
`BN_FLG_CONSTTIME`.
In this case we can instead use `src->dmax` as the number of words
allocated for `dst` and for the `memcpy` operation.

Barring compiler or runtime optimizations, if the caller provides `src`
flagged as const time and preallocated to a public size, no leak should
happen due to the copy operation.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10631)

crypto/bn/bn_lib.c

@@ -322,15 +322,19 @@ BIGNUM *BN_dup(const BIGNUM *a)
 
 BIGNUM *BN_copy(BIGNUM *a, const BIGNUM *b)
 {
+    int bn_words;
+
     bn_check_top(b);
 
+    bn_words = BN_get_flags(b, BN_FLG_CONSTTIME) ? b->dmax : b->top;
+
     if (a == b)
         return a;
-    if (bn_wexpand(a, b->top) == NULL)
+    if (bn_wexpand(a, bn_words) == NULL)
         return NULL;
 
     if (b->top > 0)
-        memcpy(a->d, b->d, sizeof(b->d[0]) * b->top);
+        memcpy(a->d, b->d, sizeof(b->d[0]) * bn_words);
 
     a->neg = b->neg;
     a->top = b->top;