Fix memory corruption caused by unchecked writes to shader_info

Mark Janes requested to merge majanes/mesa:shader_info into master

In f1dd81ae, members of the tess struct within the shader_info union were written, regardless of whether the shader was a tessellation shader.

For 64 bit builds, this memory corruption did not affect tests, perhaps due to the way the members were aligned. On 32 bit builds, thousands of tests in all suites (vulkan, deqp, piglit) crashed due to this bug.

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