video_core: Cancel Scoped's exit call on GPU failure

When CreateRenderer fails, the GraphicsContext that was std::move'd into
it is destroyed before the Scoped that was created to manage its
currency. In that case, the GraphicsContext::Scoped will still call its
destructor at the ending of the function. And because the context is
destroyed, the Scoped will cause a crash as it attempts to call a
destroyed object's DoneCurrent function.

Since we know when the call would be invalid, call the Scoped's Cancel
method. This prevents it from calling a method on a destroyed object.

src/video_core/video_core.cpp

@@ -50,6 +50,7 @@ std::unique_ptr<Tegra::GPU> CreateGPU(Core::Frontend::EmuWindow& emu_window, Cor
         gpu->BindRenderer(std::move(renderer));
         return gpu;
     } catch (const std::runtime_error& exception) {
+        scope.Cancel();
         LOG_ERROR(HW_GPU, "Failed to initialize GPU: {}", exception.what());
         return nullptr;
     }