Stop logging stack contents when reading a zero-length bandwidth file

  o Major bugfixes (directory authorities, security):

    - When directory authorities read a zero-byte bandwidth file, they log

      a warning with the contents of an uninitialised buffer. Log a warning

      about the empty file instead.

      Fixes bug 26007; bugfix on 0.2.2.1-alpha.

  time_t file_time, now;

  int ok;

  /* Initialise line, so that we can't possibly run off the end. */

  memset(line, 0, sizeof(line));

  if (fp == NULL) {

    log_warn(LD_CONFIG, "Can't open bandwidth file at configured location: %s",

             from_file);

    return -1;

  }

  if (!fgets(line, sizeof(line), fp)

          || !strlen(line) || line[strlen(line)-1] != '\n') {

  /* If fgets fails, line is either unmodified, or indeterminate. */

  if (!fgets(line, sizeof(line), fp)) {

    log_warn(LD_DIRSERV, "Empty bandwidth file");

    fclose(fp);

    return -1;

  }

  if (!strlen(line) || line[strlen(line)-1] != '\n') {

    log_warn(LD_DIRSERV, "Long or truncated time in bandwidth file: %s",

             escaped(line));

    fclose(fp);