[vulkan] Silence unimplemented format entry

src/video_core/renderer_vulkan/vk_rasterizer.cpp

@@ -1728,9 +1728,12 @@ void RasterizerVulkan::UpdateBlending(Tegra::Engines::Maxwell3D::Regs& regs) {
     if (state_tracker.TouchBlendEnable()) {
         std::array<VkBool32, Maxwell::NumRenderTargets> setup_enables{};
         for (size_t index = 0; index < Maxwell::NumRenderTargets; index++) {
-            const auto format =
-                VideoCore::Surface::PixelFormatFromRenderTargetFormat(regs.rt[index].format);
-            const bool is_integer = IsPixelFormatInteger(format);
+            bool is_integer = false;
+            if (regs.rt[index].format != Tegra::RenderTargetFormat::NONE) {
+                const auto format =
+                    VideoCore::Surface::PixelFormatFromRenderTargetFormat(regs.rt[index].format);
+                is_integer = IsPixelFormatInteger(format);
+            }
             setup_enables[index] =
                 (!is_integer && regs.blend.enable[index] != 0) ? VK_TRUE : VK_FALSE;
         }