MDEV-22043 Special character leads to assertion in my_wc_to_printable_generic on 10.5.2 (debug)

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.

sql/sql_string.cc

@@ -791,7 +791,7 @@ bool Binary_string::copy_printable_hhhh(CHARSET_INFO *to_cs,
   if (bytes_needed >= UINT_MAX32 || alloc((size_t) bytes_needed))
     return true;
   str_length= my_convert_using_func(Ptr, Alloced_length, to_cs,
-                                    my_wc_to_printable_generic,
+                                    to_cs->cset->wc_to_printable,
                                     from, from_length,
                                     from_cs,
                                     from_cs->cset->mb_wc,