[BRLTTY] BRLTTY 4.4 segfault with backtrace

[Jason White]

The circumstances which give rise to this bug appear to be:

1. The braille display is powered down.

2. Someone else logs into the machine and starts an X session. Syslog shows
that the BRLTTY segfault happens within about a minute of the start of the X
session.

3. The X session ends, I switch to another console and power up the braille
display - obviously, there's no BRLTTY process.

I had a look at api_flush() and found that quite a few locks were taken, so
it's possible that there's an undiscovered concurrency issue here.

I hope this helps to narrow down the cause. I can conduct more testing and
debugging as required.