matroskadec: Improve TTA duration calculation

Calculate the duration as accurately as possible to improve decoding of samples
where the last frame is smaller than the rest.

Signed-off-by: James Almer <jamrial@gmail.com>
Approved-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit af248fa1174200acb537a6ab1198bb2fed38e884)
avformat/matroskadec: check out_samplerate before using it in av_rescale()

Prevent assertion failure with damaged input

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 338f8b2eaf36f078eb5cc26ac10e651dc4c48243)