Avoid use-after-free if key update is initiated while ppe is pending
Avoid use-after-free if key update is initiated while ppe is pending

conn_rotate_keys() frees conn->pktns->crypto.tx.ckm. If there is a pending ppe, the ppe object still holds a reference to the freed memory and will eventually try to use it in ngtcp2_ppe_final().

ngtcp2/ngtcp2,ngtcp2/ngtcp2,ngtcp2/ngtcp2