Fix potential exception after unmounting

handleBlur() calls deferFocusInput() if the date picker is open.
deferFocusInput() schedules a later setFocus() call, which didn't
make sure the input element is still mounted before focusing it,
yielding an unhandled exception.

Author: Martin Kinkelin

src/index.jsx

@@ -220,7 +220,7 @@ export default class DatePicker extends React.Component {
   };
 
   setFocus = () => {
-    if (this.input.focus) {
+    if (this.input && this.input.focus) {
       this.input.focus();
     }
   };