Fixed issue with slow query logging where examined rows where wrong

Reset 'examined_rows_count' in union to not count same rows twice

mysql-test/r/subselect_mat_cost.result:
  Test also slow query logging
mysql-test/t/subselect_mat_cost.test:
  Test also slow query logging
sql/sql_union.cc:
  Reset 'examined_rows_count' in union to not count same rows twice

mysql-test/r/subselect_mat_cost.result

 set @subselect_mat_cost=@@optimizer_switch;
 set optimizer_switch='mrr=on,mrr_sort_keys=on,index_condition_pushdown=on';
+set log_slow_time=0.1;
 TEST GROUP 1:
 Typical cases of in-to-exists and materialization subquery strategies
 =====================================================================

mysql-test/t/subselect_mat_cost.test

@@ -15,6 +15,11 @@
 
 set @subselect_mat_cost=@@optimizer_switch;
 set optimizer_switch='mrr=on,mrr_sort_keys=on,index_condition_pushdown=on';
+#
+# Test logging to slow log (there was some errors in the log files about
+# the slow log when running under valgrind, so better to get this tested)
+#
+set log_slow_time=0.1;
 
 
 -- echo TEST GROUP 1:

sql/sql_union.cc

@@ -645,6 +645,7 @@ bool st_select_lex_unit::exec()
 	if (!saved_error)
 	{
 	  examined_rows+= thd->examined_row_count;
+          thd->examined_row_count= 0;
 	  if (union_result->flush())
 	  {
 	    thd->lex->current_select= lex_select_save;