Fonts: Fix calling ClearTexData() (clearing CPU side font data) triggering an assert in NewFrame(). (#3487) + Backends: added additional assert to facilitate detecting user understand they haven't initialized a backend.

backends/imgui_impl_dx9.cpp

@@ -362,6 +362,8 @@ void ImGui_ImplDX9_InvalidateDeviceObjects()
 void ImGui_ImplDX9_NewFrame()
 {
     ImGui_ImplDX9_Data* bd = ImGui_ImplDX9_GetBackendData();
+    IM_ASSERT(bd != NULL && "Did you call ImGui_ImplDX9_Init()?");
+
     if (!bd->FontTexture)
         ImGui_ImplDX9_CreateDeviceObjects();
 }