vk_shader_gen_spv: Return when gas mode is used

* Otherwise the shader compiler will crash since OpFunctionEnd was called

src/video_core/renderer_vulkan/vk_shader_gen_spv.cpp

@@ -71,7 +71,7 @@ void FragmentModule::Generate() {
         break;
     case TexturingRegs::FogMode::Gas:
         WriteGas();
-        break;
+        return;
     default:
         break;
     }