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/DvrClient.cpp b/media/jni/tuner/DvrClient.cpp
index 0400485..7793180 100644
--- a/media/jni/tuner/DvrClient.cpp
+++ b/media/jni/tuner/DvrClient.cpp
@@ -316,14 +316,13 @@
 Result DvrClient::close() {
     if (mTunerDvr != NULL) {
         Status s = mTunerDvr->close();
+        mTunerDvr = NULL;
         return ClientHelper::getServiceSpecificErrorCode(s);
     }
 
     if (mDvr != NULL) {
         Result res = mDvr->close();
-        if (res == Result::SUCCESS) {
-            mDvr = NULL;
-        }
+        mDvr = NULL;
         return res;
     }