MINOR: quic: Race issue when consuming RX packets buffer

MINOR: quic: Race issue when consuming RX packets buffer

Add a null byte to the end of the RX buffer to notify the consumer there is no
more data to treat.
Modify quic_rx_packet_pool_purge() which is the function which remove the
RX packet from the buffer.
Also rename this function to quic_rx_pkts_del().
As the RX packets may be accessed by the QUIC connection handler (quic_conn_io_cb())
the function responsible of decrementing their reference counters must not
access other information than these reference counters! It was a very bad idea
to try to purge the RX buffer asap when executing this function.