when the column is to be read from a derived table column which 
was specified as a concatenation of string literals.
The bug happened because the Item_string::append did not adjust the
value of Item_string::max_length. As a result of it the temporary 
table column  defined to store the concatenation of literals was 
not wide enough to hold the whole value.



mysql-test/r/subselect.result:
  Added a test case for bug #26738.
mysql-test/t/subselect.test:
  Added a test case for bug #26738.