Added cases for bugs #6307 and #6460.
sql_select.cc:
  Fixed the problem of bug reports #6307 and #6460.
  The reported wrong result sets were due to the fact that
  the added call of the fix_fields method for the built
  AND condition that joined WHERE and ON conditions
  broke ON expression, as it removed extra AND levels
  in the built condition. 
  It looks like that no attributes of the built condition
  are needed, so we don't have to call fix_fields here.