Remember channel-size to no re-read it for every read-access,
but reset it if we extend the size of the file
profiling indicates Channel.size() sometimes has similar runtime
overhead as position() or read()!
git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1877816 13f79535-47bb-0310-9956-ffa450edef68
Dominik Stadler