The AsyncRequests thread can be blocked in a SolarMutexGuard, so
we have to release the mutex before calling join().

But there is also a static AsyncRequests object inside the
VistaFilePickerEventHandler::impl_sendEvent function, which will
just be cleaned up at DLL atexit.

"Luckily" it won't ever run, so it doesn't need to be joined and
we can use isRunning to detect it, prevent releasing a non-
existing SolarMutex at this point and so won't crash LO on exit.

Change-Id: I0c56b89a11f96be54e82b756c5e18a058b04a41d
Reviewed-on: https://gerrit.libreoffice.org/66901
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>