Fix 5 issues reported by PVS studio

* src/sky.cpp  146     warn    V519 The 'suncolor_f.r' variable is assigned values twice successively. Perhaps this is a mistake. Check lines: 142, 146.
* src/sky.cpp  147     warn    V519 The 'suncolor_f.g' variable is assigned values twice successively. Perhaps this is a mistake. Check lines: 143, 147.
* src/sky.cpp  148     warn    V519 The 'suncolor_f.b' variable is assigned values twice successively. Perhaps this is a mistake. Check lines: 144, 148.
* src/threading/thread.cpp     63      err     V730 Not all members of a class are initialized inside the constructor. Consider inspecting: m_thread_obj.
* src/server.cpp       3243    err     V595 The 'log' pointer was utilized before it was verified against nullptr. Check lines: 3243, 3258.

src/threading/thread.h

@@ -152,7 +152,7 @@ private:
 	std::mutex m_mutex;
 	std::mutex m_start_finished_mutex;
 
-	std::thread *m_thread_obj;
+	std::thread *m_thread_obj = nullptr;
 
 
 #ifdef _AIX