Avoid duplicate closing on Tuner instances

set all the successfully closed instance to null
to avoid duplicate closing on the same instance.

Test: make
Bug: 176190508
Change-Id: Ie96a5de3dcc1c40a594b4bb94bf31e546a77eb40
diff --git a/media/jni/tuner/FrontendClient.cpp b/media/jni/tuner/FrontendClient.cpp
index 3a00133..4a72eba 100644
--- a/media/jni/tuner/FrontendClient.cpp
+++ b/media/jni/tuner/FrontendClient.cpp
@@ -311,15 +311,14 @@
 Result FrontendClient::close() {
     if (mTunerFrontend != NULL) {
         Status s = mTunerFrontend->close();
+        mTunerFrontend = NULL;
         return ClientHelper::getServiceSpecificErrorCode(s);
     }
 
     if (mFrontend != NULL) {
         Result result = mFrontend->close();
-        if (result == Result::SUCCESS) {
-            mFrontend = NULL;
-            mFrontend_1_1 = NULL;
-        }
+        mFrontend = NULL;
+        mFrontend_1_1 = NULL;
         return result;
     }