udev: rules - print the first invalid character

The current code would print the character following the first invalid
character.

Given an udev rules-file without a trailing newline we would otherwise print
garbage:

  invalid key/value pair in file /usr/lib/udev/rules.d/40-usb-media-players.rules
  on line 26, starting at character 25 ('m')

This is now changed to print

  invalid key/value pair in file /usr/lib/udev/rules.d/40-usb-media-players.rules
  on line 26, starting at character 25 ('')

(still not very good as printing \0 just gives the empty string)

(cherry picked from commit 6501b52d35)

src/udev/udev-rules.c

@@ -1068,13 +1068,13 @@ static int add_rule(struct udev_rules *rules, char *line,
                         /* If we aren't at the end of the line, this is a parsing error.
                          * Make a best effort to describe where the problem is. */
                         if (*linepos != '\n') {
-                                char buf[2] = {linepos[1]};
+                                char buf[2] = {*linepos};
                                 _cleanup_free_ char *tmp;
 
                                 tmp = cescape(buf);
                                 log_error("invalid key/value pair in file %s on line %u, starting at character %tu ('%s')\n",
                                           filename, lineno, linepos - line + 1, tmp);
-                                if (linepos[1] == '#')
+                                if (*linepos == '#')
                                         log_error("hint: comments can only start at beginning of line");
                         }
                         break;