bn: fix Red#imod

lib/bn.js

@@ -3169,7 +3169,10 @@
 
   Red.prototype.imod = function imod (a) {
     if (this.prime) return this.prime.ireduce(a)._forceRed(this);
-    return a.umod(this.m)._forceRed(this);
+
+    var mod = a.umod(this.m)._forceRed(this);
+    if (mod !== a) mod.copy(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);
+  });
 });