Fix (speculative) nullptr arithmetic #981

Job	Run time
Build and test cmake:gcc9	29s
Build and test cmake:clang15	32s
Build and test cmake:clang:asan	40s
Build and test cmake:clang:tsan	44s
Build and test cmake:clang:ubsan	44s
Build and test cmake:qemu-arm-neon-gcc	45s
Build and test cmake-osx:clang	27s
Build and test cmake-osx:gcc	23s
Build and test cmake-win64:msvc-rel	38s
Build and test cmake-win64:msvc-dbg	37s
Build and test fuzz:clang	37s
Build and test python3.10:clang	43s
Build and test python3.10-win	30s
Build and test maven	22s
Build and test bazel:root	39s
Build and test bazel:go	47s
Build and test bazel:java	45s
Build and test bazel:research	28s
Build and test bazel-osx:root	46s
Build and test bazel-osx:go	46s
Build and test bazel-osx:java	47s
Build and test bazel-osx:research	46s
Build and test bazel-win:root	39s
Build and test bazel-win:java	40s
Build and test bazel-win:research	39s
	15m 53s

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