Fix a null compare against an opaque struct. #148

Job	Run time
build (ubuntu-latest, Debug, address)	42s
build (ubuntu-latest, Debug, memory)	31s
build (ubuntu-latest, Debug, undefined)	26s
build (ubuntu-latest, RelWithDebInfo, address)	46s
build (ubuntu-latest, RelWithDebInfo, memory)	1m 9s
build (ubuntu-latest, RelWithDebInfo, undefined)	58s
build (ubuntu-latest, MinSizeRel, address)	38s
build (ubuntu-latest, MinSizeRel, memory)	30s
build (ubuntu-latest, MinSizeRel, undefined)	33s
build (ubuntu-latest, Release, address)	27s
build (ubuntu-latest, Release, memory)	31s
build (ubuntu-latest, Release, undefined)	39s
build (macOS-latest, Debug, address)	32s
build (macOS-latest, Debug, undefined)	45s
build (macOS-latest, RelWithDebInfo, address)	1m 8s
build (macOS-latest, RelWithDebInfo, undefined)	33s
build (macOS-latest, MinSizeRel, address)	28s
build (macOS-latest, MinSizeRel, undefined)	35s
build (macOS-latest, Release, address)	1m 2s
build (macOS-latest, Release, undefined)	1m 9s
	14m 2s

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