vulkan: Fix deref of invalid pStages pointer
The Vulkan 1.3.254 spec only requires it to be valid if VK_GRAPHICS_PIPELINE_LIBRARY_PRE_RASTERIZATION_SHADERS_BIT_EXT or VK_GRAPHICS_PIPELINE_LIBRARY_FRAGMENT_SHADER_BIT_EXT. See VUID-VkGraphicsPipelineCreateInfo-flags-06640.