bn: fix Red#imod

lib/bn.js

@@ -327,6 +327,13 @@
     dest.red = this.red;
   };
 
+  BN.prototype._move = function _move (dest) {
+    dest.words = this.words;
+    dest.length = this.length;
+    dest.negative = this.negative;
+    dest.red = this.red;
+  };
+
   BN.prototype.clone = function clone () {
     var r = new BN(null);
     this.copy(r);
@@ -3169,7 +3176,9 @@
 
   Red.prototype.imod = function imod (a) {
     if (this.prime) return this.prime.ireduce(a)._forceRed(this);
-    return a.umod(this.m)._forceRed(this);
+
+    a.umod(this.m)._forceRed(this)._move(a);
+    return a;
   };
 
   Red.prototype.neg = function neg (a) {

test/red-test.js

@@ -262,4 +262,13 @@ describe('BN.js/Reduction context', function () {
     red.prime.split(input, output);
     assert.equal(input.cmp(output), 0);
   });
+
+  it('imod should change host object', function () {
+    var red = BN.red(new BN(13));
+    var a = new BN(2).toRed(red);
+    var b = new BN(7).toRed(red);
+    var c = a.redIMul(b);
+    assert.equal(a.toNumber(), 1);
+    assert.equal(c.toNumber(), 1);
+  });
 });