This reverts commit f65afef1.

If only pts is coded then dts must be equal pts
See Rec. ITU-T H.222.0 (06/2012) / ISO/IEC 13818-1:2013 (E) 2.7.5

Found-by: Baptiste Coudurier
(cherry picked from commit 0bf88286)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>