CURRENT_USER() in GRANT ... TO CURRENT_USER() only gave us a definer,
not a full user (i.e., password-element was not initiliazed). Hence
dereferencing the password led to a crash.

Properly initializes definers now, just so there are no misunderstandings.
Also does some magic so IDENTIFIED BY ... works with CURRENT_USER().

mysql-test/r/grant2.result:
  Show GRANT ... TO CURRENT_USER() no longer crashes.
  Show it to work with IDENTIFIED BY to boot.
mysql-test/t/grant2.test:
  Show GRANT ... TO CURRENT_USER() no longer crashes.
  Show it to work with IDENTIFIED BY to boot.
sql/sql_acl.cc:
  Make IDENTIFIED BY ... work with CURRENT_USER()
sql/sql_parse.cc:
  Zero password-part of definer just in case somebody mistakes this for
  a complete LEX_USER!