avformat/mpegts: make sure mpegts_read_header always stops at the first pmt

avformat/mpegts: make sure mpegts_read_header always stops at the first pmt

mpegts_read_header stops parsing the file at the first PMT. However the check
that ensured this was wrong because streams can also be added before the first
PMT is received (e.g. EIT).

So let's make sure we are in the header reading phase by checking if ts->pkt is
unset instead of checking if the number of streams found so far is 0.

Signed-off-by: Marton Balint <cus@passwd.hu>