Problem:
In the concat method we have a case where there is a possibility of
generating corrupt output.

This is because, to check if a string is a substring of another, we
check if the substring pointer is less than or equal to the base string
pointer + its length.

The base string can span only till(less than and not equal to) its
pointer + length. If the pointer of the string being checked is
allocated immediately after the base string, it wrongly gets treated as
a substring even though it is not and hence the corruption.

Fix:
The implementation of the methods Item_func_concat::val_str() and
Item_func_concat_ws::val_str() is simplified in 8.0. Backporting the
same to avoid possibility of overlap.

Change-Id: I0ed184f41981301b2b6454865f96f6d0174de5e1