The crash happens due to wrong max_length value which is set on
Item_func_round::fix_length_and_dec() stage. The value is set to
args[0]->max_length which is too big in case of LONGTEXT(LONGBLOB) fields.
The fix is to set max_length using float_length() function.



mysql-test/r/func_math.result:
  test result
mysql-test/t/func_math.test:
  test case
sql/item_func.cc:
  The crash happens due to wrong max_length value which is set on
  Item_func_round::fix_length_and_dec() stage. The value is set to
  args[0]->max_length which is too big in case of LONGTEXT(LONGBLOB) fields.
  The fix is to set max_length using float_length() function.