Don't double destruct audio_track_cblk_t

Fortunately audio_track_cblk_t doesn't have a destructor, but for clarity
remove the double destruction.

Also add warning not to add any virtuals to audio_track_cblk_t.

Change-Id: I70ebe1a70460c7002145b2cdf10f9f137396e6f3
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index f71ba0a..a5894b6 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -3324,9 +3324,10 @@
 AudioFlinger::ThreadBase::TrackBase::~TrackBase()
 {
     if (mCblk != NULL) {
-        mCblk->~audio_track_cblk_t();   // destroy our shared-structure.
-        if (mClient == NULL) {
+        if (mClient == 0) {
             delete mCblk;
+        } else {
+            mCblk->~audio_track_cblk_t();   // destroy our shared-structure.
         }
     }
     mCblkMemory.clear();            // and free the shared memory
@@ -6970,6 +6971,7 @@
     mEffect.clear();
     if (mClient != 0) {
         if (mCblk != NULL) {
+            // unlike ~TrackBase(), mCblk is never a local new, so don't delete
             mCblk->~effect_param_cblk_t();   // destroy our shared-structure.
         }
         mCblkMemory.clear();            // and free the shared memory