Handling of large signed/unsigned values was not consistent, so some string functions could return bogus results.
The current fix is to simply patch up the val_str() methods for those string items.
It would be good clean this code up in general, to make similar problems much harder to make.  This is left as an exercise for the reader.


mysql-test/r/func_str.result:
  Update test results for bug #24947
mysql-test/t/func_str.test:
  Add test case for bug #24947
sql/item_strfunc.cc:
  Adjust some string function Items' val_str() methods to handle large signed/unsigned arguments properly