bca30570 added a user option to set max_packet_size replacing
a hardcoded value. This had a side-effect of leaving the field
set to 0 when packet demuxing is carried out from another demuxer
using avpriv functions, which could lead to demux failure.

Hardcoded max_packet_size inside avpriv_mpegts_parse_open to
2048000 to avoid this. Value chosen to be 10x that of default value
to accommodate large payloads.