The ::val_str() method has to return NULL if it calculated
        the null_value, not just set the related flag.

per-file comments:
  mysql-test/r/gis-precise.result
fix for bug #801212 Assertion with ST_INTERSECTION on NULL values
        test result updated.
  mysql-test/t/gis-precise.test
fix for bug #801212 Assertion with ST_INTERSECTION on NULL values
        test case added.
  sql/item_geofunc.cc
fix for bug #801212 Assertion with ST_INTERSECTION on NULL values
        return NULL from the val_str if we get the null_value.