Fix crash on closing the app when classifier failed to initialize

When testing on an API 21 emulator, the classifier fails to initialize.
`E/TfLiteCameraDemo: Failed to initialize an image classifier.`

In this situation, the app crashes when pressing Back to exit.  Here's the cause:
```
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.android.tflitecamerademo.ImageClassifier.close()' on a null object reference
                                                                                        at com.example.android.tflitecamerademo.Camera2BasicFragment.onDestroy(Camera2BasicFragment.java:331)
                                                                                        at android.app.Fragment.performDestroy(Fragment.java:2266)
```
The fix is to check for null before calling `.close()`.

I'll investigate why the classifier is failing to initialize separately. :-)