functions
      
String::realloc() did not check whether the existing string data fits in the newly
allocated buffer for cases when reallocating a String object with external buffer
(i.e.alloced == FALSE).  This could lead to memory overruns in some cases.


mysql-test/r/func_str.result:
  Added a test case for bug #41868.
mysql-test/t/func_str.test:
  Added a test case for bug #41868.
sql/sql_class.cc:
  After each call to Item::send() in select_send::send_data() reset buffer to its
  original state to reduce unnecessary malloc() calls. See comments for bug #41868
  for detailed analysis.
sql/sql_string.cc:
  Fixed String::realloc() to check whether the existing string data fits in the newly allocated buffer for cases when reallocating a String object with external buffer.