Bluetooth: Properly initialize UART fds

If the vendor library is unable to open the UART,
the passed in file descriptor array is unchanged.
This array was uninitialized stack memory which
may miss the illegal fd check and barrel ahead
forking a userial read thread causing FD_SET
problems should the uninitialized stack memory be
bogus fds outside the processes boundaries.
Verify that the vendor library was actually opening
exactly one uart.

Bug: 16651586

Change-Id: I24e700cdb0b0f3ed107f56b94b5b535abba66806