Correct next_line in PotentialFileReader

  int n = 0;

  int nwords = 0;

  do {

  char *ptr = fgets(line, MAXLINE, fp);

  if (ptr == nullptr) {

    // EOF

    return nullptr;

  }

  // strip comment

  if ((ptr = strchr(line, '#'))) *ptr = '\0';

  nwords = utils::count_words(line);

  if (nwords > 0) {

    n = strlen(line);

  }

  while(nwords < nparams) {

    char *ptr = fgets(&line[n], MAXLINE - n, fp);

    if (ptr == nullptr) {

      // EOF

      if (nwords > 0 && nwords < nparams) {

        char str[128];

        snprintf(str, 128, "Incorrect format in %s potential file", potential_name.c_str());

        snprintf(str, 128, "Incorrect format in %s potential file! %d/%d parameters", potential_name.c_str(), nwords, nparams);

        error->one(FLERR, str);

      }

      return nullptr;

    }

    // strip comment

    if ((ptr = strchr(line, '#'))) *ptr = '\0';

    nwords = utils::count_words(line);

    // skip line if blank

    if (nwords == 0)

      continue;

    n = strlen(line) + 1;

  } while (nwords < nparams);

    if (nwords > 0) {

      n = strlen(line);

    }

  }

  return line;

}