Fix null pointer dereference error for CtsTvTestCases against cf_x86_tv

Bug: 263933383
Test: `atest CtsTvTestCases` against cf_x86_tv-userdebug
Change-Id: Icf4f9909b574f3e6f474fd2bfd3e72f7f290a0fe
diff --git a/tv/tuner/aidl/default/Frontend.cpp b/tv/tuner/aidl/default/Frontend.cpp
index 0573d08..997d9de 100644
--- a/tv/tuner/aidl/default/Frontend.cpp
+++ b/tv/tuner/aidl/default/Frontend.cpp
@@ -182,7 +182,9 @@
     // Reset callback
     mCallback = nullptr;
     mIsLocked = false;
-    mTuner->removeFrontend(mId);
+    if (mTuner != nullptr) {
+        mTuner->removeFrontend(mId);
+    }
     mTuner = nullptr;
 
     return ::ndk::ScopedAStatus::ok();