date_format 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.

client/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.
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.