The 'decimals' member of Item_func was being improperly initialized,
  which resulted in improper results when handling large numeric values.


mysql-test/r/func_str.result:
  Add test results
mysql-test/t/func_str.test:
  Add new regression test
sql/item_strfunc.h:
  Remove unnecessary reset of decimals in Item_func_conv::fix_length_and_dec().