BN_swap_ct() use size_t for byte count

[Theo Buehler]

As pointed out by jsing, using size_t for nwords would be more
appropriate for the new internal API BN_swap_ct(). Let's switch to it
and cast to an int internally after checking the size to avoid overflow.

Index: bn/bn_lib.c
===================================================================
RCS file: /var/cvs/src/lib/libcrypto/bn/bn_lib.c,v
retrieving revision 1.44
diff -u -p -r1.44 bn_lib.c
--- bn/bn_lib.c 13 Jul 2018 08:43:31 -0000      1.44
+++ bn/bn_lib.c 14 Jul 2018 12:17:35 -0000
@@ -897,16 +897,19 @@ BN_consttime_swap(BN_ULONG condition, BI
  * nwords is the number of words to swap.
  */
 int
-BN_swap_ct(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords)
+BN_swap_ct(BN_ULONG condition, BIGNUM *a, BIGNUM *b, size_t nwords)
 {
        BN_ULONG t;
-       int i;
+       int i, words;
 
        if (a == b)
                return 1;
-       if (bn_wexpand(a, nwords) == NULL || bn_wexpand(b, nwords) == NULL)
+       if (nwords > INT_MAX)
                return 0;
-       if (a->top > nwords || b->top > nwords) {
+       words = (int)nwords;
+       if (bn_wexpand(a, words) == NULL || bn_wexpand(b, words) == NULL)
+               return 0;
+       if (a->top > words || b->top > words) {
                BNerror(BN_R_INVALID_LENGTH);
                return 0;
        }
@@ -930,7 +933,7 @@ BN_swap_ct(BN_ULONG condition, BIGNUM *a
        b->flags ^= t;
 
        /* swap the data */
-       for (i = 0; i < nwords; i++) {
+       for (i = 0; i < words; i++) {
                t = (a->d[i] ^ b->d[i]) & condition;
                a->d[i] ^= t;
                b->d[i] ^= t;
Index: bn/bn_lcl.h
===================================================================
RCS file: /var/cvs/src/lib/libcrypto/bn/bn_lcl.h,v
retrieving revision 1.28
diff -u -p -r1.28 bn_lcl.h
--- bn/bn_lcl.h 10 Jul 2018 21:52:07 -0000      1.28
+++ bn/bn_lcl.h 14 Jul 2018 12:15:51 -0000
@@ -606,7 +606,7 @@ BIGNUM *BN_mod_inverse_nonct(BIGNUM *ret
 int    BN_gcd_ct(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx);
 int    BN_gcd_nonct(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx);
 
-int    BN_swap_ct(BN_ULONG swap, BIGNUM *a, BIGNUM *b, int nwords);
+int    BN_swap_ct(BN_ULONG swap, BIGNUM *a, BIGNUM *b, size_t nwords);
 
 __END_HIDDEN_DECLS
 #endif