ByteArraySeekableByteChannel.position|truncate(long) shouldn't throw an IllegalArgumentException for a new positive position that's too large

[garydgregory]

ByteArraySeekableByteChannel's position(long) and truncate(long) shouldn't throw an
IllegalArgumentException for a new positive position that's too large.

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[ppkarwasz]

LGTM, but we can add two small improvements.