flterm eats 100% of the CPU when the serial adapter is unplugged

**How to reproduce the issue:**

 * `python -m lambdasoc.tools.flterm /dev/ttyUSB1` (`/dev/ttyUSB1` being a valid serial device)
 * Unplug the USB-serial adapter from the computer

**Expected behaviour:**

flterm quits when the serial device isn't available anymore