Fix possibly invalid pointer #447

Job	Run time
mmg-debug / ci (ubuntu-20.04, on, off, on, off, int32_t)	27m 52s
mmg-debug / ci (ubuntu-20.04, on, off, off, off, int32_t)	31m 53s
mmg-debug / ci (ubuntu-20.04, off, off, on, off, int32_t)	32m 44s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int32_t)	33m 3s
mmg-debug / ci (macos-12, on, off, on, off, int32_t)	1s
mmg-debug / ci (macos-12, on, off, off, off, int32_t)	1s
mmg-debug / ci (macos-12, off, off, on, off, int32_t)	39s
mmg-debug / ci (macos-12, off, off, off, off, int32_t)	5m 47s
mmg-debug / ci (ubuntu-20.04, off, off, off, on, int32_t)	5m 21s
mmg-debug / ci (macos-12, off, off, off, on, int32_t)	1s
mmg-debug / ci (ubuntu-20.04, off, on, on, off, int32_t)	29m 17s
mmg-debug / ci (macos-12, off, on, on, off, int32_t)	1s
mmg-debug / ci (windows-2022, off, off, off, off, int32_t)	24m 20s
mmg-debug / ci (windows-2022, on, off, off, off, int32_t)	19m 57s
mmg-debug / ci (windows-2022, off, on, off, off, int32_t)	23m 51s
mmg-debug / ci (ubuntu-20.04, on, on, on, on, int64_t)	5m 59s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int64_t)	3m 32s
mmg-debug / ci (macos-12, on, on, on, on, int64_t)	1s
mmg-debug / ci (macos-12, off, off, off, off, int64_t)	5m 9s
mmg-debug / ci (windows-2022, on, on, off, off, int64_t)	5m 11s
mmg-debug / ci (windows-2022, off, off, off, off, int64_t)	4m 52s
	4h 19m 32s

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