COLUMN ARE REVEALED

Issue:
-----
When a hexadecimal representation of a string literal is
passed as a parameter to the insert function, additional
information is displayed by the SELECT statement.


SOLUTION:
---------
This happens because while creating the hexadecimal
character, the "Alloced_length" is set to the string
length, but the actual allocation does not happen. This
will result in the same string being used for multiple
rows, and the new string will be appended to the old one.

The solution is to check whether a string is actually
allocated, if not make sure that this is done.

Also, when a string is supplied from a variable,
String->realloc will result in truncation if 'to' and
'from' overlap. This needs to be handled by forcing an
allocation on the heap.

Functions like lcase/encode/decode may return substrings
that are already allocated on the heap. concat/concat_ws
can have similar problems where temporary results are
over-written. Here uses_buffer_owned_by() can be used
to check if the input string is already allocated on the
heap. If yes, a temporary variable is used to store the
substring.

This fix is a backport of Bug#11765149, Bug#20315088 and
Bug#20554017.