The problem is that array binding uses net buffer to read parameters for each
execution while each execiting with RETURNING write in the same buffer.

Solution is to allocate new net buffer to avoid changing buffer we are reading
from.

The code did not take into account that:
- U+005C (backslash) can occupy more than mbminlen characters (e.g. in sjis)
- Some character sets do not have a code for U+005C (e.g. swe7)

Adding a new function my_wc_to_printable into MY_CHARSET_HANDLER to
cover all special cases easier.

restore check for client charset_info == NULL, which was previously
removed in MDEV-8844

Now both offset and limit are stored and do not chenged during execution
(offset is decreased during processing in versions before 10.5).

(Big part of this changes made by Monty)