fix(datepicker): make sure _datepickerInput exists before accessing its

disabled property.

Author: mmalerba

src/lib/datepicker/datepicker.ts

@@ -148,7 +148,8 @@ export class MdDatepicker<D> implements OnDestroy {
   /** Whether the datepicker pop-up should be disabled. */
   @Input()
   get disabled() {
-    return this._disabled === undefined ? this._datepickerInput.disabled : this._disabled;
+    return this._disabled === undefined && this._datepickerInput ?
+        this._datepickerInput.disabled : this._disabled;
   }
   set disabled(value: any) {
     const newValue = coerceBooleanProperty(value);