MDEV-26817 runtime error: index 24320 out of bounds for type 'json_string_char_classes [128] *and* ASAN: global-buffer-overflow on address ... READ of size 4 on SELECT JSON_VALID

protect from out-of-bound array access

it was already done in all other places, this one was the only one missed

strings/json_lib.c

@@ -826,7 +826,7 @@ static int skip_key(json_engine_t *j)
 {
   int t_next, c_len;
 
-  if (json_instr_chr_map[j->s.c_next] == S_BKSL &&
+  if (j->s.c_next<128 && json_instr_chr_map[j->s.c_next] == S_BKSL &&
       json_handle_esc(&j->s))
     return 1;