This warning come from a copy() operation of type:
memcpy(ptr, ptr+A, B), which is safe but produces a warning
when run with valgrind.

To avoid the warning, I added copy_or_move() method which uses
memmove() instead of memcpy().

In 10.3 the change in item_strfunc::Item_func_concat() has to be mirroed
in Item_func_concat_oracle() to avoid future valgrind warnings.