avoid undefined behavior from UNREACHABLE on reachable code paths (#7… #993

Job	Run time
Mac (Debug, clang, x86_64)	11m 50s
Mac (Release, clang, x86_64)	15m 51s
Mac (Release, clang, arm64)	11m 13s
Mac (Debug, clang, arm64)	12m 11s
Linux (Release, gcc-13, x86_64)	17m 6s
Linux (Debug, gcc-9, x86_64)	16m 29s
Linux (Release, clang-16, x86_64)	22m 47s
Linux (Debug, gcc-13, x86_64)	13m 0s
Linux (Release, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	16m 6s
Linux (Debug, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	12m 20s
Linux (Debug, clang-16, x86_64)	11m 14s
Linux (Release, gcc-9, x86_64)	16m 23s
	2h 56m 30s

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