fpspreadsheet: Fix issue 0025133 "Cannot read xls file apparently BIFF5". Fix error message when writing a too-long cell text to BIFF5.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3653 8e941d3f-bd1b-0410-a28a-d453659cc2b4