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>

hw/dmx/dmxsync.c

@@ -182,7 +182,7 @@ dmxSync(DMXScreenInfo * dmxScreen, Bool now)
 
         /* Do sync or set time for later */
         if (now || !dmxScreen) {
-            if (!TimerForce(dmxSyncTimer))
+            if (dmxSyncTimer == NULL || !TimerForce(dmxSyncTimer))
                 dmxSyncCallback(NULL, 0, NULL);
             /* At this point, dmxSyncPending == 0 because
              * dmxSyncCallback must have been called. */