The result of a ^ 0 mod -1 is 0 not 1

Thanks to Guido Vranken and OSSFuzz for finding this issue.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6355)

crypto/bn/bn_exp.c

@@ -178,8 +178,8 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
 
     bits = BN_num_bits(p);
     if (bits == 0) {
-        /* x**0 mod 1 is still zero. */
-        if (BN_is_one(m)) {
+        /* x**0 mod 1, or x**0 mod -1 is still zero. */
+        if (BN_abs_is_word(m, 1)) {
             ret = 1;
             BN_zero(r);
         } else {
@@ -320,8 +320,8 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
     }
     bits = BN_num_bits(p);
     if (bits == 0) {
-        /* x**0 mod 1 is still zero. */
-        if (BN_is_one(m)) {
+        /* x**0 mod 1, or x**0 mod -1 is still zero. */
+        if (BN_abs_is_word(m, 1)) {
             ret = 1;
             BN_zero(rr);
         } else {
@@ -629,8 +629,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
      */
     bits = p->top * BN_BITS2;
     if (bits == 0) {
-        /* x**0 mod 1 is still zero. */
-        if (BN_is_one(m)) {
+        /* x**0 mod 1, or x**0 mod -1 is still zero. */
+        if (BN_abs_is_word(m, 1)) {
             ret = 1;
             BN_zero(rr);
         } else {
@@ -1143,8 +1143,8 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
 
     bits = BN_num_bits(p);
     if (bits == 0) {
-        /* x**0 mod 1 is still zero. */
-        if (BN_is_one(m)) {
+        /* x**0 mod 1, or x**0 mod -1 is still zero. */
+        if (BN_abs_is_word(m, 1)) {
             ret = 1;
             BN_zero(rr);
         } else {
@@ -1265,8 +1265,8 @@ int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
 
     bits = BN_num_bits(p);
     if (bits == 0) {
-        /* x**0 mod 1 is still zero. */
-        if (BN_is_one(m)) {
+        /* x**0 mod 1, or x**0 mod -1 is still zero. */
+        if (BN_abs_is_word(m, 1)) {
             ret = 1;
             BN_zero(r);
         } else {