Fix a bunch of crashes discovered by fuzzing cppfront #869

Job	Run time
ubu-24 \| g++-14 \| c++2b \| libstdc++ \| ubuntu-24.04	6m 16s
ubu-24 \| g++-13 \| c++2b \| libstdc++ \| ubuntu-24.04	5m 36s
ubu-20 \| g++-10 \| c++20 \| libstdc++ \| ubuntu-20.04	1s
ubu-24 \| clang++-18 \| c++20 \| libstdc++ \| ubuntu-24.04	7m 22s
ubu-24 \| clang++-18 \| c++23 \| libc++-18-dev \| ubuntu-24.04	7m 31s
ubu-22 \| clang++-15 \| c++20 \| libstdc++ \| ubuntu-22.04	5m 53s
ubu-22 \| clang++-15 \| c++20 \| libc++-15-dev \| ubuntu-22.04	5m 45s
ubu-20 \| clang++-12 \| c++20 \| libstdc++ \| ubuntu-20.04	1s
mac-14 \| clang++ \| c++2b \| default \| macos-14	2m 19s
mac-13 \| clang++ \| c++2b \| default \| macos-13	5m 19s
mac-13 \| clang++-15 \| c++2b \| default \| macos-13	8m 49s
win-22 \| cl.exe \| c++latest \| default \| windows-2022	3m 25s
win-22 \| cl.exe \| c++20 \| default \| windows-2022	3m 25s
aggregate-results	0s
	1h 1m 42s

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