[FEThumbnails]Add cleanup code to avoid locking files (#32108)

src/modules/previewpane/PdfThumbnailProviderCpp/PdfThumbnailProvider.cpp

@@ -182,6 +182,13 @@ IFACEMETHODIMP PdfThumbnailProvider::GetThumbnail(UINT cx, HBITMAP* phbmp, WTS_A
         }
     }
 
+    // ensure releasing the stream (not all if branches contain it)
+    if (m_pStream)
+    {
+        m_pStream->Release();
+        m_pStream = NULL;
+    }
+
     return S_OK;
 }