Check userial fd before reading

The userial read loop uses one global flag
to condition the serial port read and a
second global file descriptor variable that
may change from another thread underneath it.

Check the validity of the serial port file
descriptor by making a copy immediately before
the read and only proceeding if it's a valid fd.

bug: 17552690

Change-Id: Iaaf1b8476fdee462ddc178d2a31ef8f55cb33e52