An assertion abort could occur for some grouping queries that employed 
decimal user variables with assignments to them.

The problem appeared the constructors of the class Field_new_decimal
because the function my_decimal_length_to_precision did not guarantee
returning decimal precision not greater than DECIMAL_MAX_PRECISION.


mysql-test/r/type_newdecimal.result:
  Added a test case for bug #29417.
mysql-test/t/type_newdecimal.test:
  Added a test case for bug #29417.
sql/field.cc:
  Fixed bug #29417.
  An assertion abort could occur for some grouping queries that employed 
  decimal user variables with assignments to them.
  
  The problem appeared the constructors of the class Field_new_decimal
  because the function my_decimal_length_to_precision did not guarantee
  returning decimal precision not greater than DECIMAL_MAX_PRECISION.
  
  Now if the precision returned by calls to my_decimal_length_to_precision
  in the constructors of the class Field_new_decimal is greater than 
  DECIMAL_MAX_PRECISION the precision is set to this value.