Bluetooth: Check the number of file descriptors

Opening the serial port(s) returns -1 when it fails, check
that the number of file descriptors is greater than 0 and
less than the size of the array.

Test: VtsHalBluetoothV1_0TargetTest, Bluetooth start/stop
Bug: 64420262
Change-Id: Icbffa5bd8c02030e389bac03aa5e1e78cc3c6057
1 file changed