Release clock only when one is set.

diff --git a/dlls/quartz/videorenderer.c b/dlls/quartz/videorenderer.c
index fd738c4..e3de4fb 100644
--- a/dlls/quartz/videorenderer.c
+++ b/dlls/quartz/videorenderer.c
@@ -422,7 +422,9 @@
     if (!refCount)
     {
         DeleteCriticalSection(&This->csFilter);
-        IReferenceClock_Release(This->pClock);
+
+        if (This->pClock)
+            IReferenceClock_Release(This->pClock);
         
         IPin_Release(This->ppPins[0]);