The ROUND(X, D) function would change the Item::decimals field during
execution to achieve the effect of a dynamic number of decimal digits.
This caused a series of bugs:
Bug #30617:Round() function not working under some circumstances in InnoDB
Bug #33402:ROUND with decimal and non-constant cannot round to 0 decimal places
Bug #30889:filesort and order by with float/numeric crashes server
Fixed by never changing the number of shown digits for DECIMAL when
used with a nonconstant number of decimal digits.


mysql-test/r/type_decimal.result:
  Bug#33143: Test result
mysql-test/t/type_decimal.test:
  Bug#33143: Test case
sql/item_func.cc:
  Bug#33143: 
  - Moved the DECIMAL_MAX_SCALE limitation to fix_length_and_dec.
  - Removed resetting of Item::decimals field.
  - set the frac field of the output value to current scale.
strings/decimal.c:
  Bug#33143: It is necessary to set all digits in the buffer following the 
  rounded one to zero, as they may now be displayed.