lavc/bitstream_filter: do not crash in case the argument of av_bitstream_filter_close() is NULL
lavf: fix documentation for avoid_negative_ts

Prefer "non-negative" over "positive", the former is more accurate.
mpegts: only reopen pmt_cb filter if its different from the previous.

Fixes Ticket2632

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