Fix memory corruption and assert failures in convex decomposition #16751

Job	Run time
📊 Static checks / Code style, file formatting, and docs	34s
🤖 Android / Template (target=template_release)	27m 17s
🐧 Linux / Editor w/ Mono (target=editor)	32m 48s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	34m 52s
🍏 iOS / Template (target=template_release)	14m 34s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	30m 20s
🍎 macOS / Editor (target=editor, tests=yes)	1h 16m 12s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	25m 45s
🏁 Windows / Editor (target=editor, tests=yes)	39m 9s
🍎 macOS / Template (target=template_release)	19m 24s
🐧 Linux / Template w/ Mono (target=template_release)	21m 54s
🏁 Windows / Template (target=template_release)	27m 54s
🐧 Linux / Minimal template (target=template_release, everything disabled)	13m 38s
🌐 Web / Template (target=template_release)	18m 25s
🪲 Godot CPP / Build and test Godot CPP	6m 51s
	6h 29m 37s

Provide feedback