mfplat/tests: Fix buffer object leak (Valgrind).

Signed-off-by: Nikolay Sivov <nsivov@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/mfplat/tests/mfplat.c

@@ -1712,6 +1712,8 @@ todo_wine
 todo_wine
     ok(count == 1, "Unexpected buffer count %u.\n", count);
 
+    IMFMediaBuffer_Release(buffer);
+
     IMFSample_Release(sample);
 }