MINOR: quic: Add a BUG_ON() on quic_tx_packet refcount

MINOR: quic: Add a BUG_ON() on quic_tx_packet refcount

This is definitively a bug to call quic_tx_packet_refdec() to decrement the reference
counter of a TX packet calling quic_tx_packet_refdec(), and possibly to release its
memory when it is negative or null.

This counter is incremented when a TX frm is attached to it with some allocated memory
and when the packet is inserted into a data structure, if needed (list or tree).

Should be easily backported as far as 2.6 to ease any further backport around
this code part.