journalctl: replace string_erase with memset('x')

The compiler should not be able to optimize out the memset, because optarg is global
memory. In this case, not making the argument an empty string is nicer, so just use
an open-coded version of string_erase from before the explicit_bzero change.

src/journal/journalctl.c

@@ -686,7 +686,9 @@ static int parse_argv(int argc, char *argv[]) {
                         r = free_and_strdup(&arg_verify_key, optarg);
                         if (r < 0)
                                 return r;
-                        string_erase(optarg);
+                        /* Use memset not string_erase so this doesn't look confusing
+                         * in ps or htop output. */
+                        memset(optarg, 'x', strlen(optarg));
 
                         arg_merge = false;
                         break;