Corruption when using small backing buffers

[JosephCottam]

If backing an IndexedEncoding with a BigFileByteBuffer and a small buffer, the indexed encoding can return incorrect values IF the buffer offset is moved.

Only affects BigFileByteBuffer because its the only one that repositions the buffer. Bug is rare because FileByteBuffer utilities will return FileByteBuffer objects when requested segment is small enough to fit in a standard buffer.