Merge "boot_control: Don't allow current_slot_ to be -1."

  // was not set (from either the command line or the device tree), we can later

  // initialize it from the bootloader_control struct.

  std::string suffix_prop = android::base::GetProperty("ro.boot.slot_suffix", "");

  if (suffix_prop.empty()) {

    LOG(ERROR) << "Slot suffix property is not set";

    return false;

  }

  current_slot_ = SlotSuffixToIndex(suffix_prop.c_str());

  std::string err;