Backport of #809: fix crash in `Descriptor::parse_desc` #2198

Job	Run time
Test - 1.56.1 toolchain (minimal)	26m 53s
Test - nightly toolchain (minimal)	13m 28s
Test - nightly toolchain (recent)	13m 7s
Docs - nightly toolchain (recent)	58s
Test - stable toolchain (recent)	14m 43s
Test - stable toolchain (minimal)	14m 43s
Bench - nightly toolchain (recent)	58s
Format - nightly toolchain	16s
Embedded	3m 47s
Integration tests	33s
Lint - nightly toolchain (recent)	34s
Docs - stable toolchain (recent)	35s
	1h 30m 35s

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