Optimize xml_buffered_writer::write(char_t...)

Make it easier for the compiler to generate good code by loading bufsize
into a local once and returning new offset from flush(). This results in
7% performance gain.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1024 99668b35-9821-0410-8761-19e4c4f06640