Backends: avoid null dereference in metal and osx shutdown (#6385, #6334)

Co-authored-by: Alexander Rath <alex@ist.besonders.cool>

backends/imgui_impl_metal.mm

@@ -137,12 +137,13 @@ void ImGui_ImplMetal_Shutdown()
 {
     ImGui_ImplMetal_Data* bd = ImGui_ImplMetal_GetBackendData();
     IM_ASSERT(bd != nullptr && "No renderer backend to shutdown, or already shutdown?");
+    ImGui_ImplMetal_DestroyDeviceObjects();
+    ImGui_ImplMetal_DestroyBackendData();
+
     ImGuiIO& io = ImGui::GetIO();
     io.BackendRendererName = nullptr;
     io.BackendRendererUserData = nullptr;
     io.BackendFlags &= ~ImGuiBackendFlags_RendererHasVtxOffset;
-    ImGui_ImplMetal_DestroyDeviceObjects();
-    ImGui_ImplMetal_DestroyBackendData();
 }
 
 void ImGui_ImplMetal_NewFrame(MTLRenderPassDescriptor* renderPassDescriptor)

backends/imgui_impl_osx.mm

@@ -477,7 +477,6 @@ void ImGui_ImplOSX_Shutdown()
 {
     ImGui_ImplOSX_Data* bd = ImGui_ImplOSX_GetBackendData();
     IM_ASSERT(bd != nullptr && "No platform backend to shutdown, or already shutdown?");
-    ImGuiIO& io = ImGui::GetIO();
 
     bd->Observer = nullptr;
     if (bd->Monitor != nullptr)
@@ -486,10 +485,12 @@ void ImGui_ImplOSX_Shutdown()
         bd->Monitor = nullptr;
     }
 
+    ImGui_ImplOSX_DestroyBackendData();
+
+    ImGuiIO& io = ImGui::GetIO();
     io.BackendPlatformName = nullptr;
     io.BackendPlatformUserData = nullptr;
     io.BackendFlags &= ~(ImGuiBackendFlags_HasMouseCursors | ImGuiBackendFlags_HasGamepad);
-    ImGui_ImplOSX_DestroyBackendData();
 }
 
 static void ImGui_ImplOSX_UpdateMouseCursor()