nouveau: add returns following assert(0)

If asserts are compiled out, we reach the end of a non-void function
and return garbage.

Signed-off-by: Maya Rashish <coypu@sdf.org>
22 jobs for !1854 with assert-returns in 53 minutes and 33 seconds (queued for 1 minute and 23 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed debian #563054

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  Build+Test
passed meson-arm64 #563063

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passed meson-armhf #563062

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passed meson-clang #563057

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passed meson-clover #563060

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passed meson-i386 #563064

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passed meson-main #563055

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passed meson-swr-glvnd #563056

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passed meson-vulkan #563061

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passed scons-llvm #563066

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passed scons-nollvm #563065

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passed scons-swr #563058

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passed scons-win64 #563059

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  Test
passed test-llvmpipe-gles2 1/4 #563067

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passed test-llvmpipe-gles2 2/4 #563068

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passed test-llvmpipe-gles2 3/4 #563069

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passed test-llvmpipe-gles2 4/4 #563070

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passed test-softpipe-gles2 1/4 #563071

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passed test-softpipe-gles2 2/4 #563072

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passed test-softpipe-gles2 3/4 #563073

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passed test-softpipe-gles2 4/4 #563074

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passed test-softpipe-gles3-limited #563075

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