Never return -1 from BN_exp

Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1455)

crypto/bn/bn_exp.c

@@ -46,7 +46,7 @@ int BN_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0) {
         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
         BNerr(BN_F_BN_EXP, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
-        return -1;
+        return 0;
     }
 
     BN_CTX_start(ctx);
@@ -177,7 +177,7 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0) {
         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
         BNerr(BN_F_BN_MOD_EXP_RECP, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
-        return -1;
+        return 0;
     }
 
     bits = BN_num_bits(p);
@@ -1123,7 +1123,7 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0) {
         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
         BNerr(BN_F_BN_MOD_EXP_MONT_WORD, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
-        return -1;
+        return 0;
     }
 
     bn_check_top(p);
@@ -1254,7 +1254,7 @@ int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0) {
         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
         BNerr(BN_F_BN_MOD_EXP_SIMPLE, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
-        return -1;
+        return 0;
     }
 
     bits = BN_num_bits(p);