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.

strings/ctype-bin.c

@@ -560,6 +560,7 @@ static MY_CHARSET_HANDLER my_charset_handler=
   my_well_formed_char_length_8bit,
   my_copy_8bit,
   my_wc_mb_bin,
+  my_wc_to_printable_generic
 };