The bug appears after implementation of WL#2984
(Make stored routine variables work according to the standard).


mysql-test/r/type_varchar.result:
  Update result file.
mysql-test/t/type_varchar.test:
  Add a test for BUG#15588.
sql/field.cc:
  - use memmove() instead of memcpy() -- after implementation of WL#2984
    (Make stored routine variables work according to the standard) it is
    possible to store in the field the value from this field. For instance,
    this can happen for the following statement:
      SET sp_var = SUBSTR(sp_var, 1, 3);
sql/sp_head.cc:
  - Work correctly with String:
    - String length has to be be reset before use;
    - qs_append() does not allocate memory, so the memory should
      be reserved beforehand.
sql/sql_select.cc:
  Polishing: should have been done in WL#2984.