crash in command_interpreter() with part of Unicode-characters

  /* special case to handle one-character, non-alphanumeric commands; requested
   * by many people so "'hi" or ";godnet test" is possible. Patch sent by Eric
   * Green and Stefan Wasilewski. */
  if (!isalpha(*argument)) {
    arg[0] = argument[0];
    arg[1] = '\0';
    line = argument + 1;
  } else
    line = any_one_arg(argument, arg);

interpreter.c: In function ‘command_interpreter’:
interpreter.c:488:35: warning: initialization of ‘char’ from ‘char *’ makes integer from pointer without a cast [-Wint-conversion]
  488 |   const char specialSingleChars = "*';";
      |                                   ^~~~~
interpreter.c:500:14: warning: passing argument 1 of ‘strchr’ makes pointer from integer without a cast [-Wint-conversion]
  500 |   if (strchr(specialSingleChars, *argument) != NULL) {
      |              ^~~~~~~~~~~~~~~~~~
      |              |
      |              char
In file included from sysdep.h:74,
                 from interpreter.c:12:
/usr/include/string.h:226:14: note: expected ‘const char *’ but argument is of type ‘char’
  226 | extern char *strchr (const char *__s, int __c)
      |              ^~~~~~