Fix two off-by-one correctness issues in string transcoding (#12886)

The first issue fixes here is that the maximum string byte length was
one byte larger than the spec defines (`1<<31` instead of `

Fix two off-by-one correctness issues in string transcoding (#12886)

The first issue fixes here is that the maximum string byte length was
one byte larger than the spec defines (`1<<31` instead of `(1<<31)-1`).
The second issue is that in the comparison against the maximum byte
length `>=` was used where `>` should have been used. A test has been
added which exercises these two cases.

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