quartz: Ensure pSample is NULL before calling IAsyncReader_WaitForNext

[Bruno Jesus]

This is already done in 2 of 3 places, if WaitForNext fails pSample
could be left uninitialized.
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diff --git a/dlls/quartz/pin.c b/dlls/quartz/pin.c
index 8377be5..52a3970 100644
--- a/dlls/quartz/pin.c
+++ b/dlls/quartz/pin.c
@@ -415,6 +415,7 @@ static void PullPin_Flush(PullPin *This)
         {
             DWORD_PTR dwUser;
 
+            pSample = NULL;
             IAsyncReader_WaitForNext(This->pReader, 0, &pSample, &dwUser);
 
             if (!pSample)