x/crypto: add ReadUint32LengthPrefixed

[AlexanderYastrebov]

Fixes golang/go#49227

[gopherbot]

This PR (HEAD: 838991638a7f798354f9748bc3f12218557ea615) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/360095 to see it.

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