BUG/MEDIUM: quic: break out of the loop in quic_lstnr_dghdlr

BUG/MEDIUM: quic: break out of the loop in quic_lstnr_dghdlr

The function processes packets sent by other threads in the current
thread's queue. But if, for any reason, other threads write faster
than the current one processes, this can lead to a situation where
the function never returns.

It seems that it might be what's happening in issue #1808, though
unfortunately, this function is one of the rare without traces. But
the amount of calls to functions like qc_lstnr_pkt_rcv() on a single
thread seems to indicate this possibility.

Thanks to Tristan for his efforts in collecting extremely precious
traces!

This likely needs to be backported to 2.6.