Fix gcc-4.8 compilation warning when using -Wstrict-overflow

git-svn-id: http://pugixml.googlecode.com/svn/trunk@956 99668b35-9821-0410-8761-19e4c4f06640
Fix gcc-4.9 compilation warning when using -Wstrict-overflow

git-svn-id: http://pugixml.googlecode.com/svn/trunk@957 99668b35-9821-0410-8761-19e4c4f06640