AutoPtr: do 'duplicate' before 'release'

Common knowledge in reference counting is "on assignment increment first
then decrement", because "just to be deleted" object could hold last
reference to "just to be assigned" one.

Fixes #3979

Foundation/include/Poco/AutoPtr.h

@@ -110,34 +110,23 @@ class AutoPtr
 	{
 		if (_ptr != ptr)
 		{
-			if (_ptr) _ptr->release();
-			_ptr = ptr;
-			if (shared && _ptr) _ptr->duplicate();
+			if (shared && ptr) ptr->duplicate();
+			std::swap(_ptr, ptr);
+			if (ptr) ptr->release();
 		}
 		return *this;
 	}
 
 	AutoPtr& assign(const AutoPtr& ptr)
 	{
-		if (&ptr != this)
-		{
-			if (_ptr) _ptr->release();
-			_ptr = ptr._ptr;
-			if (_ptr) _ptr->duplicate();
-		}
-		return *this;
+		return assign(ptr._ptr, true);
 	}
 
 	template <class Other>
 	AutoPtr& assign(const AutoPtr<Other>& ptr)
 	{
-		if (ptr.get() != _ptr)
-		{
-			if (_ptr) _ptr->release();
-			_ptr = const_cast<Other*>(ptr.get());
-			if (_ptr) _ptr->duplicate();
-		}
-		return *this;
+		C* nptr = const_cast<Other*>(ptr.get());
+		return assign(nptr, true);
 	}
 
 	void reset()