Fix "applying non-zero offset 4 to null pointer" UB

[vitalybuka]

Detected with

cmake .. -GNinja \
  -DCMAKE_C_FLAGS="-fsanitize=pointer-overflow -fno-sanitize-trap=pointer-overflow -fno-sanitize-recover=pointer-overflow" \
  -DCMAKE_CXX_FLAGS="-fsanitize=pointer-overflow -fno-sanitize-trap=pointer-overflow -fno-sanitize-recover=pointer-overflow" \
  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=pointer-overflow" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++

ninja
ninja test

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[schwehr]

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