When the ORDER BY clause gets fixed it's allowed to search in the current
item_list in order to find aliased fields and expressions. This is ok for a
SELECT but wrong for an UPDATE statement. If the ORDER BY clause will
contain a non-existing field which is mentioned in the UPDATE set list
then the server will crash due to using of non-existing (0x0) field.

When an Item_field is getting fixed it's allowed to search item list for
aliased expressions and fields only for selects.


sql/sql_base.cc:
  Bug#25126: Wrongly resolved field leads to a crash.
  When an Item_field is getting fixed it's allowed to search item list for
  aliased expressions and fields only for selects.
sql/sql_select.cc:
  Bug#25126: Wrongly resolved field leads to a crash.
  When an Item_field is getting fixed it's allowed to search item list for
  aliased expressions and fields only for selects.
mysql-test/r/update.result:
  Added a test case for bug#25126: Wrongly resolved field leads to a crash.
mysql-test/t/update.test:
  Added a test case for bug#25126: Wrongly resolved field leads to a crash.