fix crashing in campaign tree in FRED #5099

Job	Run time
Windows (Debug, MSVC, Win32)	16m 48s
Windows (Debug, MSVC, x64)	17m 17s
Mac (Debug, clang, x86_64)	6m 58s
Linux (Debug, gcc-9)	9m 46s
Mac (Debug, clang, arm64)	8m 19s
Windows (Release, MSVC, Win32)	17m 52s
Linux (Debug, gcc-13)	2m 10s
Mac (Release, clang, x86_64)	7m 59s
Linux (Debug, clang-16)	9m 15s
Windows (Release, MSVC, x64)	20m 32s
Mac (Release, clang, arm64)	7m 37s
Linux (Release, gcc-9)	12m 51s
Linux (Release, gcc-13)	11m 14s
Linux (Release, clang-16)	14m 40s
Linux (Debug, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	8m 14s
Linux (Release, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	6m 50s
	2h 58m 22s

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