It looks like line 256's strchr("_-.", str[y]) != NULL) is
backwards.  You want to execute the conditional if str[y] is
not one of "_-.", in which case strchr() returns NULL.

url.c:255-260 are:

#else /*CHARSET_EBCDIC*/
} else if (!isalnum(str[y]) && strchr("_-.", str[y]) != NULL) {
  str[y++] = '%';
  str[y++] = hexchars[os_toascii[(unsigned char) s[x]] >> 4];
  str[y] = hexchars[os_toascii[(unsigned char) s[x]] & 0x0F];
}
#endif /*CHARSET_EBCDIC*/