LocalFileAdapter::skip() works incorrect in a very rare case

You may use skip() function to return character to the steam (skip(-nChars)). When you read the block again, the reading must start from -nChars offset.

But in the following case the function doesn't work.

LocalFileAdapter has BUF_SIZE variable. Suppose BUF_SIZE=30.

//pseudo code
{
io->readBlock(buf, 30);
//30 first chars in the internal buffer

io->readBlock(buf, 30);
//30 next chars in the internal buffer

io->skip(-35);
//we cannot seek to the first buffer block because it's lost
//and the next reading will not start from -35 offset
}

The bug came from "SNP project".