Fixed TACHYON-97

Current implementation of read(long position, byte[] buffer, int offset, int length) in HdfsFileInputStream is incomplete: 
1. It does not consider the boundary of file length, which could lead to an out of range issue when seeking the requested position in bytebuffer. 
2. It does not recover the original offset after function call.
The NPE exception should be returned to the caller to this method. I didn't carefully examine the exception part from original submitter, and will submit another improved version soon.