Fix buffer size boundary check offset by 1

When newPos == file size, the current block will not be read or Get. If this block is a crucial part of the document (like m_pTrailer), the program will exit with parse error and
the document will not be rendered.

BUG=None
R=jun_fang@foxitsoftware.com

Review URL: https://codereview.chromium.org/440563003