Backtrace logging etc. is already sigsafe, but the actual FatalError message
in response is not yet, leading to amusing logs like this:

    (EE) Segmentation fault at address 0x0
    (EE) BUG: triggered 'if (inSignalContext)'
    (EE) BUG: log.c:499 in LogVMessageVerb()
    (EE) Warning: attempting to log data in a signal unsafe manner while in
    signal context.
    Please update to check inSignalContext and/or use LogMessageVerbSigSafe() or
    ErrorFSigSafe().
    The offending log format message is:

    Fatal server error:

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 042c6d86)