Fixed undefined state after Reset/Inject call

The instance's Emplace and SetExtern methods called the dtor, which could set the instance into an undefined state.
Moved cleanup from the dtor function into a dedicated method to avoid the dtor's object-lifetime ending effect.

# Conflicts:
#	include/singleton.hpp

Author: Sp3EdeR

include/singleton.hpp

@@ -64,9 +64,7 @@ struct Singleton
         Instance(const Instance&) = delete;
         ~Instance()
         {
-            // Destroys the locally-initialized instance. Injected ones are ignored (no ownership).
-            if (m_pExtern == LOCAL_INSTANCE_ID)
-                GetBuffer().~T();
+            DestroyLocalInstance();
         }
         Instance& operator =(const Instance&) = delete;
         /// Returns the current instance.
@@ -75,7 +73,7 @@ struct Singleton
         template <typename ...Args>
         void Emplace(Args... args)
         {
-            this->~Instance();
+            DestroyLocalInstance();
             new (&GetBuffer()) T(std::forward<Args>(args)...);
             m_pExtern = LOCAL_INSTANCE_ID;
         }
@@ -84,7 +82,7 @@ struct Singleton
           */
         void SetExtern(T* ptr)
         {
-            this->~Instance();
+            DestroyLocalInstance();
             m_pExtern = ptr;
         }
     private:
@@ -100,6 +98,13 @@ struct Singleton
             static union U { T asT; U(){} ~U(){} } buffer;
             return buffer.asT;
         }
+        /// Destroys the local instance, while ignoring injected ones (no ownership).
+        /** @remark This leaves the object in an inconsistent state. m_pExtern must be changed. */
+        void DestroyLocalInstance()
+        {
+            if (m_pExtern == LOCAL_INSTANCE_ID)
+                GetBuffer().~T();
+        }
     } g_instance;
 
     /// Holds a flag used for `std::call_once()`.