avformat/mpegts: Only start probing data streams within probe_packets

Fixes assertion failure
Fixes: 4321db8ac331f5967ebfbfe80ce5eb78/signal_sigabrt_7ffff6ae7cc9_7213_0d6457b9d6897fa7c78507fa5de53510.ts

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