avformat/movenc: Check first DTS similar to dts difference

Fixes assertion failure
Fixes: b84b53855a0b74560e64c6f45f505a13/signal_sigabrt_7ffff6ae7c37_3837_ef4e243ea5b4fa8d0becf4afe9166604.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
