turnip/pipeline: Don't assume tu_shader is a valid object

Fixes a crash in tu6_emit_fs_config() when 'shader' argument is
assumed to be non-null, which is possible.

Fixes dEQP test:

dEQP-VK.api.descriptor_set.descriptor_set_layout_lifetime.graphics

Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Tested-by: Marge Bot <!3756>
Part-of: <!3756>
40 jobs for master in 41 minutes and 29 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Container
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aarch64
arm_build

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aarch64
arm_test

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x86_build

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x86_build_old

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x86_test-gl

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  Build
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aarch64
meson-arm64

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aarch64
meson-armhf

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meson-clang

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meson-clover

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meson-clover-old-llvm

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meson-i386

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meson-main

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meson-mingw32-x86_64

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gstreamer
meson-ppc64el

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gstreamer
meson-s390x

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meson-testing

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meson-vulkan

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scons

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scons-old-llvm

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scons-win64

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  Test
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db410c
arm64_a306_gles2

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mesa-cheza
arm64_a630_gles2

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mesa-cheza
arm64_a630_gles3

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mesa-cheza
arm64_a630_gles31

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llvmpipe-traces

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lava-sun50i-h6-pine-h64
panfrost-t720-test:arm64

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lava-rk3288-veyron-jaq
panfrost-t760-test:armhf

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lava-rk3399-gru-kevin
panfrost-t860-test:arm64

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piglit-glslparser

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piglit-quick_gl

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piglit-quick_shader

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test-llvmpipe-gles2

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test-softpipe-gles2

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test-softpipe-gles31 1/4

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test-softpipe-gles3 1/2

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test-softpipe-gles31 2/4

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test-softpipe-gles31 3/4

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test-softpipe-gles3 2/2

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