mysql-test/r/bigint.result:
  Test to show show that the parser threats big longlong values as unsigned
mysql-test/r/user_var.result:
  Test of CAST(NULL as SIGNED/UNSIGNED)
mysql-test/t/bigint.test:
  Test to show show that the parser threats big longlong values as unsigned
mysql-test/t/user_var.test:
  Test of CAST(NULL as SIGNED/UNSIGNED)
sql/item_func.cc:
  Setting a variable to CAST(NULL as X) set the result type of the variable to X. (Bug #6598)
  Setting a variable to NULL doesn't change the old result type.
sql/item_func.h:
  Detect setting a variable to NULL
sql/unireg.cc:
  Safety fix