avformat/asfenc: Check pts

Fixes integer overflow
Fixes: 0063df8be3aaa30dd6d76f59c8f818c8/signal_sigsegv_7b7b59_3634_bf418b6822bbfa68734411d96b667be3.mov

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