matroska_read_seek: Fix used streams for subtitle index compensation

Might fix Ticket1907 (I have no testcase so i cant test)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 4758e32a6c48044f77102a49110c79b4f338f648)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>