Check the return value when reading HCI type byte

Add missing return value check when reading the HCI type byte.
This check is needed as a safeguard. For example, function
event_uart_has_bytes() could be called (indirectly)
within the run_reactor() loop not only when there are bytes to read,
but also if there is an error (e.g., EPOLLHUP | EPOLLRDHUP | EPOLLERR).

Bug: 23105107
Change-Id: Ic3b6e4d656406949e384c8106b0c607f7c221759