[AD] double signal hang fix

[Vincent Penquerc'h <Vincent.Penquerch@xxxxxxxxxx>]

This fixes a possible hang when recovering from a fatal signal,
and allegro_exit also fires such a one.
I'm not sure why it's needed, as the signal routine should have
been unregistered after being called - maybe it's only unregistered
after it has terminated, I'm not sure what POSIX says (if it's
POSIX). Only tested with Linux console, so the QNX/X/DOS versions
might be unnecessary/wrong/whatever.

(The hang is actually an infinite recursion, and my machine went
on to crawling, I didn't wait for a OOM kill, which I guess would
have happened).

-- 
Vincent Penquerc'h

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