(temporary) TABLE, crash

Problem: if one has an open "HANDLER t1", further "TRUNCATE t1" 
doesn't close the handler and leaves handler table hash in an 
inconsistent state, that may lead to a server crash.

Fix: TRUNCATE should implicitly close all open handlers.

Doc. request: the fact should be described in the manual accordingly.


mysql-test/r/handler_myisam.result:
  Fix for bug #46456 [Ver->Prg]: HANDLER OPEN + TRUNCATE + DROP
  (temporary) TABLE, crash
    - test result.
mysql-test/t/handler_myisam.test:
  Fix for bug #46456 [Ver->Prg]: HANDLER OPEN + TRUNCATE + DROP
  (temporary) TABLE, crash
    - test case.
sql/sql_delete.cc:
  Fix for bug #46456 [Ver->Prg]: HANDLER OPEN + TRUNCATE + DROP
   (temporary) TABLE, crash
    - remove all truncated tables from the HANDLER's hash.