lavf: enable av_dlog() in compute_pkt_fields2()

Turns a comment into an av_dlog() instruction, also add a commented
issues.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 77f21ce4)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>