Fix crash in ConfirmCredentials due to missing fp service.

Fixes bug 27060816

Change-Id: I25fa1ffc65247360f58473f5bc1745abbabb1e33
(cherry picked from commit 0918e5d6)

src/com/android/settings/fingerprint/FingerprintUiHelper.java

@@ -49,7 +49,8 @@ public class FingerprintUiHelper extends FingerprintManager.AuthenticationCallba
     }
 
     public void startListening() {
-        if (mFingerprintManager.getEnrolledFingerprints(mUserId).size() > 0) {
+        if (mFingerprintManager != null
+                && mFingerprintManager.getEnrolledFingerprints(mUserId).size() > 0) {
             mCancellationSignal = new CancellationSignal();
             mFingerprintManager.setActiveUser(mUserId);
             mFingerprintManager.authenticate(