When parsing a field literal containing two Huffman-encoded strings,
don't decode the first string until verifying all data is present.
Avoids forced quadratic complexity when repeatedly parsing a partial
field, repeating the Huffman decoding of the string on each iteration.

Thanks to Philippe Antoine (Catena cyber) for reporting this issue.

Fixes golang/go#57855
Fixes CVE-2022-41723

Change-Id: I58a743df450a4a4923dddd5cf6bb0592b0a7bdf3
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1688184
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Reviewed-on: https://go-review.googlesource.com/c/net/+/468135
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