amstream: Initialize correctly AM_MEDIA_TYPE struct.

dlls/amstream/mediastreamfilter.c

@@ -132,6 +132,9 @@ static HRESULT WINAPI BasePinImp_GetMediaType(BasePin *This, int index, AM_MEDIA
     MSPID purpose_id;
     int i;
 
+    /* FIXME: Reset structure as we only fill majortype and minortype for now */
+    ZeroMemory(amt, sizeof(*amt));
+
     /* Find which stream is associated with the pin */
     for (i = 0; i < filter->nb_streams; i++)
         if (&This->IPin_iface == filter->pins[i])