Corr copy assigment operator, as suggested by clang-tidy cert-oop54-cpp

Sample/Material/Material.cpp

@@ -31,6 +31,8 @@ Material::Material(const Material& material)
 
 Material& Material::operator=(const Material& other)
 {
+    if(this == &other)
+        return *this;
     ASSERT(!other.isEmpty());
     m_material_impl.reset(other.m_material_impl->clone());
     return *this;