MINOR: quic: Replace pool_zalloc() by pool_malloc() for fake datagrams

MINOR: quic: Replace pool_zalloc() by pool_malloc() for fake datagrams

These fake datagrams are only used by the low level I/O handler. They
are not provided to the "by connection" datagram handlers. This
is why they are not MT_LIST_APPEND()ed to the listner RX buffer list
(see &quic_dghdlrs[cid_tid].dgrams in quic_lstnr_dgram_dispatch().
Replace the call to pool_zalloc() to by the lighter call to pool_malloc()
and initialize only the ->buf and ->length members. This is safe because
only these fields are inspected by the low level I/O handler.