Fix CircularByteBuffer.peek() matching bytes past the buffered data

[lenamonj]

CircularByteBuffer.peek() returned false early only when length was smaller than the number of buffered bytes. A larger length fell through to the comparison loop, which read past the valid bytes into whatever the backing array held there, so a fresh buffer holding {1, 2} reported an exact match for {1, 2, 0, 0}. Through PeekableInputStream.peek() a two-byte stream matched a four-byte pattern, and a scanner branching on that result took the branch for content the stream does not contain.

The guard now returns false unless length equals the buffered count, the exact-match rule IO-855 documented.

Tests: testPeekLongerThanBuffered and testPeekLongerThanInputDoesNotMatch, red on master and green with the change. Default Maven goal green.

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