cborparser_dup_string: don't modify *buffer until success #20

Job	Run time
ubuntu-latest/gcc-no-math	58s
ubuntu-latest/gcc-freestanding	40s
ubuntu-latest/clang	1m 12s
ubuntu-latest/linux-g++	1m 13s
macos-13/clang	2m 7s
	6m 10s

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