BUG/MINOR: quic: Wrong list_for_each_entry() use when building packets from qc_do_build_pkt()

This is list_for_each_entry_safe() which must be used if we want to delete elements
inside its code block. This could explain that some frames which were not built were added
to packets with a NULL ->pkt member.

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

Must be backported to 2.6.
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