dmx: Fix null pointer dereference

A null pointer dereference can occur in dmxSync, because TimerForce
does not handle a null pointer.

dmxSyncTimer is set to NULL a few lines above on a certain condition,
which happened on my machine. The explicit NULL check allowed me to
start Xdmx again without a segmentation fault.

Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 21eda746)