bn/bn_intern.c: const-ify bn_set_{static}_words.

Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6738)

crypto/bn/bn_intern.c

@@ -172,16 +172,20 @@ BN_ULONG *bn_get_words(const BIGNUM *a)
     return a->d;
 }
 
-void bn_set_static_words(BIGNUM *a, BN_ULONG *words, int size)
+void bn_set_static_words(BIGNUM *a, const BN_ULONG *words, int size)
 {
-    a->d = words;
+    /*
+     * |const| qualifier omission is compensated by BN_FLG_STATIC_DATA
+     * flag, which effectively means "read-only data".
+     */
+    a->d = (BN_ULONG *)words;
     a->dmax = a->top = size;
     a->neg = 0;
     a->flags |= BN_FLG_STATIC_DATA;
     bn_correct_top(a);
 }
 
-int bn_set_words(BIGNUM *a, BN_ULONG *words, int num_words)
+int bn_set_words(BIGNUM *a, const BN_ULONG *words, int num_words)
 {
     if (bn_wexpand(a, num_words) == NULL) {
         BNerr(BN_F_BN_SET_WORDS, ERR_R_MALLOC_FAILURE);

crypto/include/internal/bn_int.h

@@ -47,7 +47,7 @@ BN_ULONG *bn_get_words(const BIGNUM *a);
  * Set the internal data words in a to point to words which contains size
  * elements. The BN_FLG_STATIC_DATA flag is set
  */
-void bn_set_static_words(BIGNUM *a, BN_ULONG *words, int size);
+void bn_set_static_words(BIGNUM *a, const BN_ULONG *words, int size);
 
 /*
  * Copy words into the BIGNUM |a|, reallocating space as necessary.
@@ -58,7 +58,7 @@ void bn_set_static_words(BIGNUM *a, BN_ULONG *words, int size);
  * |num_words| is int because bn_expand2 takes an int. This is an internal
  * function so we simply trust callers not to pass negative values.
  */
-int bn_set_words(BIGNUM *a, BN_ULONG *words, int num_words);
+int bn_set_words(BIGNUM *a, const BN_ULONG *words, int num_words);
 
 /*
  * Some BIGNUM functions assume most significant limb to be non-zero, which