Fixed crash with NzContext::Destroy

Fixed crash when calling NzContext::Destroy two times without recreating
a new context

src/Nazara/Renderer/Context.cpp

@@ -191,6 +191,7 @@ void NzContext::Destroy()
 
 		m_impl->Destroy();
 		delete m_impl;
+		m_impl = nullptr;
 	}
 }