Prevent invalid columnPos() call in formUpdateBuffer()

Bug-Debian: https://github.com/tats/w3m/issues/89
form_write_data(FILE * f, char *boundary, char *name, char *value)
{
    fprintf(f, "--%s\r\n", boundary);
    fprintf(f, "Content-Disposition: form-data; name=\"%s\"\r\n\r\n", name);
    fprintf(f, "%s\r\n", value);
}

form_write_data(FILE * f, char *boundary, char *name, char *value)
{
    fprintf(f, "--%s\r\n", boundary);
    fprintf(f, "Content-Disposition: form-data; name=\"%s\"\r\n\r\n", name);
    fprintf(f, "%s\r\n", value);
}
