Registration of pointer change if we assign it to other pointer which should be identical after statement execution (PS/SP).

mysql-test/r/subselect.result:
  Test suite.
mysql-test/t/subselect.test:
  Test suite.
sql/sql_class.cc:
  The procedure of the pointer registration.
sql/sql_class.h:
  The procedure of the pointer registration.
sql/sql_lex.cc:
  Registration of pointer change if we assign it to other pointer which should be identical after statement execution (PS/SP).