Fix LoginEmailFragment NPE crash in onActivityResult()

[AmandaRiu]

Fixes #408

The Issue

The crash is around the email address hint functionality on the email field for login. A NPE is thrown in the onActiivtyResult() method of the LoginEmailFragment. The mEmailInput variable is null and the code is attempting to call a method on this variable in onActivityResult():

public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == EMAIL_CREDENTIALS_REQUEST_CODE) {
        if (resultCode == RESULT_OK) {
            Credential credential = data.getParcelableExtra(Credential.EXTRA_KEY);
            mEmailInput.getEditText().setText(credential.getId());
            next(getCleanedEmail());
        } else {
            mHasDismissedEmailHints = true;
            mEmailInput.getEditText().postDelayed(new Runnable() { <------ CRASHES HERE -----|
                @Override
                public void run() {
                    if (isAdded()) {
                        ActivityUtils.showKeyboard(mEmailInput.getEditText());
                    }
                }
            }, getResources().getInteger(android.R.integer.config_mediumAnimTime));
        }

        mIsDisplayingEmailHints = false;
    }
}

mEmailInput gets initialized in the setupContent() method which is called during onCreateView() in the base Fragment class. So knowing this I'm assuming that somehow LoginEmailFragment's onActivityResult() is being called before onCreateView(). I've tried to replicate this crash by enabling "Don't keep activities" and trying to somehow hit that sweet spot where I activate the TextInput and then put the app in the background before the hint results are returned. No luck.

The Fix

Verify mEmailInput is not null before attempting to access it. There is no side effect of this fix since when setupContent() is finally called and mEmailInput is initialized, the email hint code will be called again.

[nbradbury]