bn: fix iaddn sign issue. see indutny/bn.js#216.

Author: chjj

lib/js/bn.js

@@ -186,7 +186,7 @@ class BN {
 
     // Possible sign change.
     if (this.negative !== 0) {
-      if (this.length === 1 && (this.words[0] | 0) < num) {
+      if (this.length === 1 && (this.words[0] | 0) <= num) {
         this.words[0] = num - (this.words[0] | 0);
         this.negative = 0;
         return this;

test/bn-test.js

@@ -411,6 +411,11 @@ describe('BN.js', function() {
           new BN(0).iaddn(0x4000000);
         });
       });
+
+      it('should reset sign if value equal to value in instance', function () {
+        const a = new BN(-1);
+        assert.equal(a.addn(1).toString(), '0');
+      });
     });
 
     describe('.sub()', () => {