When issuing a column level grant on a table which require pre-locking the 
server crashed.

The reason behind the crash was that data structures used by the lock api
wasn't properly reinitialized in the case of a column level grant.


mysql-test/r/grant.result:
  * Added test case
mysql-test/t/grant.test:
  * Added test case
sql/sql_acl.cc:
  * The lock api is dending on the thd->lex object and this variable needs to 
    be re-initialized when opened with a new set of tables than specified in the
    original statement.