Fix a null compare against an opaque struct. #147

Job	Run time
build (ubuntu-latest, Debug, address)	32s
build (ubuntu-latest, Debug, memory)	38s
build (ubuntu-latest, Debug, undefined)	25s
build (ubuntu-latest, RelWithDebInfo, address)	51s
build (ubuntu-latest, RelWithDebInfo, memory)	41s
build (ubuntu-latest, RelWithDebInfo, undefined)	42s
build (ubuntu-latest, MinSizeRel, address)	36s
build (ubuntu-latest, MinSizeRel, memory)	27s
build (ubuntu-latest, MinSizeRel, undefined)	33s
build (ubuntu-latest, Release, address)	39s
build (ubuntu-latest, Release, memory)	1m 1s
build (ubuntu-latest, Release, undefined)	38s
build (macOS-latest, Debug, address)	36s
build (macOS-latest, Debug, undefined)	35s
build (macOS-latest, RelWithDebInfo, address)	48s
build (macOS-latest, RelWithDebInfo, undefined)	1m 15s
build (macOS-latest, MinSizeRel, address)	57s
build (macOS-latest, MinSizeRel, undefined)	38s
build (macOS-latest, Release, address)	32s
build (macOS-latest, Release, undefined)	36s
	13m 40s

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