Fix writeFully argument validation in Builder.kt

[BenWoodworth]

override fun writeFully(src: IntArray, offset: Int, length: Int) {
    require(length >= 0) { "length shouldn't be negative: $length" }
    require(offset + length < src.lastIndex) { "offset ($offset) + length ($length) >= src.lastIndex (${src.lastIndex})" }
    // ...

The second require statement was incorrect in the writeFully methods. (e.g. for src=[1], offset=0, length=1, you'll get require(0 + 1 < 0))

I also added validation to make sure offset is non-negative, like in other places in the repo.

[BenWoodworth]

I'm not sure why it's failing CI checks (compiles fine and passes tests on my machine), but let me know if any changes need to be made

[qwwdfsad]

Closing as "won't fix", please see https://github.com/Kotlin/kotlinx-io#obsolete-warning