precision > 0 && scale <= precision'.

A sign of a resulting item of the IFNULL function was not
updated and the maximal length of this result was calculated
improperly. Correct algorithm was copy&pasted from the IF
function implementation.


sql/item_cmpfunc.cc:
  Fixed bug #31471.
  The Item_func_ifnull::fix_length_and_dec method has been
  modified to update the Item_func_ifnull::unsigned_flag field
  and to take this field into account when calculating the
  Item_func_ifnull::max_length value.
  (See Item_func_if::fix_length_and_dec for reference).
mysql-test/t/null.test:
  Added test case for bug #31471.
mysql-test/r/null.result:
  Added test case for bug #31471.
mysql-test/r/create.result:
  Update test case after the bugfix of bug #31471.