Fix for bug #14536: SELECT @A,@A:=... fails with prepared statements


mysql-test/r/func_sapdb.result:
  Correct tests after reverting patch for BUG #14009 (use of abs() on null value causes problems with filesort)
mysql-test/r/type_newdecimal.result:
  Correct tests after reverting patch for BUG #14009 (use of abs() on null value causes problems with filesort)
mysql-test/r/user_var.result:
  More test with SELECT @A:=
mysql-test/t/disabled.def:
  Enable user_var.test for
mysql-test/t/user_var.test:
  More test with SELECT @A:=
sql/item.cc:
  Simple optimization during review of new code
sql/item_func.cc:
  Reverting patch for BUG #14009 (use of abs() on null value causes problems with filesort)
sql/item_timefunc.h:
  timediff() can return NULL for not NULL arguments
sql/sql_base.cc:
  Remove usage of current_thd() in mysql_make_view()
sql/sql_lex.h:
  Remove usage of current_thd() in mysql_make_view()
sql/sql_select.cc:
  Fix for bug #14536: SELECT @A,@A:=... fails with prepared statements
sql/sql_view.cc:
  Remove usage of current_thd() in mysql_make_view()
  Simple optimization of new code
sql/sql_view.h:
  Remove usage of current_thd() in mysql_make_view()
sql/table.cc:
  Simple optimization of new code