fpspreadsheet: Fix arithmetic overflow in ooxml writer in case of empty worksheet but existing row records. Use try-finally blocks around test cases to make sure that memory is released and temp file is deleted in case of exceptions due to failed tests. Fix memory leak in rpn formula calculation (spreadtestgui, however, still reports a lot of memory leaks). 

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3431 8e941d3f-bd1b-0410-a28a-d453659cc2b4