backport(4.x): fix imaskn state (#317)

Author: fanatid

lib/bn.js

@@ -2129,6 +2129,11 @@
       this.words[this.length - 1] &= mask;
     }
 
+    if (this.length === 0) {
+      this.words[0] = 0;
+      this.length = 1;
+    }
+
     return this.strip();
   };
 

test/binary-test.js

@@ -56,6 +56,8 @@ describe('BN.js/Binary', function () {
 
   describe('.imaskn()', function () {
     it('should mask bits in-place', function () {
+      assert.equal(new BN(42).imaskn(0).toString(), '0');
+      assert.equal(new BN(42).imaskn(0).toString(16), '0');
       assert.equal(new BN(0).imaskn(1).toString(16), '0');
       assert.equal(new BN(3).imaskn(1).toString(16), '1');
       assert.equal(new BN('123456789', 16).imaskn(4).toString(16), '9');