build: slightly change files to pass compilation

Compiled using Clang and encountered errors on ambiguous pointer
type, implicit conversion NULL to bool, and unused function, etc.
As a fix, this patch slightly updates the related codes.

Change-Id: I4a73d66293313dda36749036f5c73aaf2a445231

src/core/tracker.h

@@ -83,7 +83,7 @@ class Tracker {
 
   static void Destroy() {
     std::lock_guard<mutex_t> lck(glob_mutex_);
-    if (instance_ != NULL) delete instance_;
+    if (instance_ != NULL) delete instance_.load();
     instance_ = NULL;
   }