message: Fix ObjectBase's move constructor not setting handle to nullptr
  This caused a crash because if handle was set to garbage then the
  move-constructor would try to call the Release function on it.
diff: @@ -58,8 +58,7 @@ namespace nxt {
  template<typename Derived, typename CType>
  class ObjectBase {
  public:
  - ObjectBase(): handle(nullptr) {
  - }
  + ObjectBase() = default;
  ObjectBase(CType handle): handle(handle) {
  if (handle) Derived::NxtReference(handle);
  }
  @@ -71,14 +70,13 @@ namespace nxt {
  Derived& operator=(ObjectBase const& other) = delete;
  ObjectBase(ObjectBase&& other) {
  - Derived::NxtRelease(handle);
  handle = other.handle;
  other.handle = 0;
  }
  Derived& operator=(ObjectBase&& other) {
  if (&other == this) return static_cast<Derived&>(*this);
  - Derived::NxtRelease(handle);
  + if (handle) Derived::NxtRelease(handle);
  handle = other.handle;
  other.handle = 0;
  @@ -106,7 +104,7 @@ namespace nxt {
  }
  protected:
  - CType handle;
  + CType handle = nullptr;
  };
  {% macro render_cpp_method_declaration(type, method) %}
  