represented by an expression of the type UNSIGNED INT and this 
expression was evaluated to 0 then the function erroneously returned
the value of the first argument instead of an empty string. 

This problem was introduced by the patch for bug 10963.

The problem has been resolved by a proper modification of the code of
Item_func_substr::val_str.


mysql-test/r/func_str.result:
  Added a test case for bug #27130.
mysql-test/t/func_str.test:
  Added a test case for bug #27130.