Currently the Last_query_cost session status variable shows
only the cost of a single flat subselect. For complex queries
(with subselects or unions etc) Last_query_cost is not valid
as it was showing the cost for the last optimized subselect.
Fixed by reseting to zero Last_query_cost when the complete
cost of the query cannot be determined.
Last_query_cost will be non-zero only for single flat queries.


mysql-test/r/status.result:
  Bug #30377: test case
mysql-test/t/status.test:
  Bug #30377: test case
sql/sql_lex.h:
  Bug #30377: helper function
sql/sql_select.cc:
  Bug #30377: don't assign cost if not on single level statement