Fix a null compare against an opaque struct. #141

Job	Run time
build (ubuntu-latest, Debug, address)	25s
build (ubuntu-latest, Debug, memory)	23s
build (ubuntu-latest, Debug, undefined)	22s
build (ubuntu-latest, RelWithDebInfo, address)	37s
build (ubuntu-latest, RelWithDebInfo, memory)	29s
build (ubuntu-latest, RelWithDebInfo, undefined)	43s
build (ubuntu-latest, MinSizeRel, address)	30s
build (ubuntu-latest, MinSizeRel, memory)	29s
build (ubuntu-latest, MinSizeRel, undefined)	37s
build (ubuntu-latest, Release, address)	32s
build (ubuntu-latest, Release, memory)	33s
build (ubuntu-latest, Release, undefined)	33s
build (macOS-latest, Debug, address)	24s
build (macOS-latest, Debug, undefined)	35s
build (macOS-latest, RelWithDebInfo, address)	35s
build (macOS-latest, RelWithDebInfo, undefined)	58s
build (macOS-latest, MinSizeRel, address)	56s
build (macOS-latest, MinSizeRel, undefined)	1m 10s
build (macOS-latest, Release, address)	29s
build (macOS-latest, Release, undefined)	45s
	12m 5s

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