Use mutex when reading prepared_stmt_count global status variable.

Update test case for bug 16365 and bug 23159: add test for
prepared_stmt_count being decreased when some connection that had
prepared statements is closed.


mysql-test/r/ps.result:
  Update result for bug#16365 and bug#23159.
mysql-test/t/ps.test:
  Update test case for bug#16365 and bug#23159: add test for
  prepared_stmt_count being decreased when some connection that had
  prepared statements is closed.
sql/mysqld.cc:
  Use mutex when reading prepared_stmt_count global status variable.