fix: prevent calling addEventListener on non-functions

This was throwing exceptions in firefox for the following case:
```javascript
elt.onclick = null;
```

test/zone.spec.js

@@ -143,6 +143,17 @@ describe('Zone.patch', function () {
       expect(log).toEqual('b');
     });
 
+    it('should handle removing onclick', function () {
+      var log = '';
+      button.onclick = function () {
+        log += 'a';
+      };
+      button.onclick = null;
+
+      button.click();
+      expect(log).toEqual('');
+    });
+
   });
 
   describe('hooks', function () {

zone.js

@@ -106,9 +106,13 @@ Zone.patchProperty = function (obj, prop) {
       this.removeEventListener(eventName, this[_prop]);
     }
 
-    this[_prop] = fn;
 
-    this.addEventListener(eventName, fn, false);
+    if (typeof fn === 'function') {
+      this[_prop] = fn;
+      this.addEventListener(eventName, fn, false);
+    } else {
+      this[_prop] = null;
+    }
   };
 
   desc.get = function () {