backport(4.x): Fix imuln/muln with zero (backport of #313) (#314)

Co-authored-by: CanardMandarin <thibault.marboud@gmail.com>

Author: fanatid

lib/bn.js

@@ -1921,6 +1921,7 @@
       this.words[i] = carry;
       this.length++;
     }
+    this.length = num === 0 ? 1 : this.length;
 
     return this;
   };

test/arithmetic-test.js

@@ -290,6 +290,16 @@ describe('BN.js/Arithmetic', function () {
         new BN(0).imuln(0x4000000);
       }, /^Error: Assertion failed$/);
     });
+
+    it('should strip length on zero multiplication', function () {
+      var a = new BN(42);
+      assert.equal(a.clone().imuln(0).isZero(), true);
+      assert.equal(a.clone().muln(0).isZero(), true);
+
+      var b = new BN('1222222225255589');
+      assert.equal(b.clone().imuln(0).isZero(), true);
+      assert.equal(b.clone().muln(0).isZero(), true);
+    });
   });
 
   describe('.pow()', function () {