Clarify verify_checksum guards

}

verify_checksum() {

  if [ -z "$HAS_MD5_SUPPORT" ]; then

    return 0

  fi

  # If there's no MD5 support, return success

  [ -n "$HAS_MD5_SUPPORT" ] || return 0

  # If the specified filename doesn't exist, return failure

  local filename="$1"

  if [ ! -e "$filename" ]; then

    return 1

  fi

  [ -e "$filename" ] || return 1

  # If there's no expected checksum, return success

  local expected_checksum="$2"

  if [ -z "$expected_checksum" ]; then

    return 0

  fi

  [ -n "$expected_checksum" ] || return 0

  # If the computed checksum is empty, return failure

  local computed_checksum="$(compute_md5 < "$filename")"

  if [ -z "$computed_checksum" ]; then

    return 1

  fi

  [ -n "$computed_checksum" ] || return 1

  if [ "$expected_checksum" != "$computed_checksum" ]; then

    { echo