subqueries on information schema that use MIN/MAX aggregation.

Execution of some correlated subqueries may set the value
of null_row to 1 for tables used in the subquery.
If the the subquery is on information schema it causes
rejection of any row for the following executions of
the subquery in the case when an optimization filtering
by some condition is applied.

The fix restores the value of the null_row flag for 
each execution of a subquery on information schema.


mysql-test/r/information_schema.result:
  Added a test case for bug #18925.
mysql-test/t/information_schema.test:
  Added a test case for bug #18925.
sql/sql_show.cc:
  Fixed bug #18925.
  Execution of some correlated subqueries may set the value
  of null_row to 1 for tables used in the subquery.
  If the the subquery is on information schema it causes
  rejection of any row for the following execitions of
  the subquery in the case when an optimization filtering
  by some condition is applied.
  
  The fix restores the value of the null_row flag for 
  each execution of a subquery on information schema.