Line loops weren't recognized when collect results.
    Fixed by checking if we got the same beginning point of the line.

per-file comments:
  mysql-test/r/gis-precise.result
bug 839341 100% CPU usage with ST_UNION in maria-5.3-gis.
      test result updated.
  mysql-test/t/gis-precise.test
bug 839341 100% CPU usage with ST_UNION in maria-5.3-gis.
      test case added.
  sql/gcalc_tools.cc
bug 839341 100% CPU usage with ST_UNION in maria-5.3-gis.
      check if we get the beginning node of the linestring, then cut the loop.