BUG/MINOR: quic: Null packet dereferencing from qc_dup_pkt_frms() trace

This function must duplicate frames be resent from packets. Some of
them are still in flight, others have already been detected as lost.
In this case the original frame ->pkt member is NULL.
Add a trace to distinguish these cases.

Thank you to Tristan for having reported this issue in GH #1808.

Must be backported to 2.6.
1 file changed