BUG/MINOR: quic: fix buffer overflow with sockaddr_in46

BUG/MINOR: quic: fix buffer overflow with sockaddr_in46

New type sockaddr_in46 has been recently introduced. It serves as a
union which can store either an IPv4 or IPv6 address. The objective is
to reduce the storage size for QUIC datagrams which previously uses a
sockaddr_storage field.

On qc_new_conn(), source and destination addresses from the datagram are
passed to the function as sockaddr_storage so that they are copied into
the newly built quic_conn instance. However, the involved memcpy() is
producing a buffer overflow as sockaddr_in46 is smaller than
sockaddr_storage type.

This patch fixes this by defining a new helper function
in46un_to_addr(). This allows to convert safely sockaddr_in46 to a plain
sockaddr type. The function is now used before invoking qc_new_conn().

Note that there is still other several places where union sockaddr_in46
is casted as sockaddr_storage type. However, these should be safe as in
these cases sockaddr fields are accessed individually after checking
ss_family. The memory issue only exists when plain memcpy is used.

This bug was detected using ASAN. It generates the following traces when
a QUIC connection is instantiated.

==37474==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7c3bb9a61100 at pc 0x5631f52c3946 bp 0x7ffc83e45b50 sp 0x7ffc83e45310
READ of size 128 at 0x7c3bb9a61100 thread T0
    #0 0x5631f52c3945 in __asan_memcpy (/home/amaury/work/haproxy-quic-dev/haproxy+0x3ae945) (BuildId: a7ccfd74b7a71a869b8ff8d13f6dcde8c82c1487)
    #1 0x5631f55f9e34 in qc_new_conn /home/amaury/work/haproxy-quic-dev/src/quic_conn.c:1311:2
    #2 0x5631f558d5c3 in quic_rx_pkt_retrieve_conn /home/amaury/work/haproxy-quic-dev/src/quic_rx.c:1875:10
    #3 0x5631f558330b in quic_dgram_parse /home/amaury/work/haproxy-quic-dev/src/quic_rx.c:2463:29
    #4 0x5631f5625da6 in quic_lstnr_dghdlr /home/amaury/work/haproxy-quic-dev/src/quic_sock.c:206:3
    #5 0x5631f6a64173 in run_tasks_from_lists /home/amaury/work/haproxy-quic-dev/src/task.c:660:26
    #6 0x5631f6a6ba1e in process_runnable_tasks /home/amaury/work/haproxy-quic-dev/src/task.c:913:9
    #7 0x5631f5e984c3 in run_poll_loop /home/amaury/work/haproxy-quic-dev/src/haproxy.c:2982:3
    #8 0x5631f5e9a715 in run_thread_poll_loop /home/amaury/work/haproxy-quic-dev/src/haproxy.c:3212:2
    #9 0x5631f5e9f732 in main /home/amaury/work/haproxy-quic-dev/src/haproxy.c:3853:2
    #10 0x7f2bba8276c0  (/usr/lib/libc.so.6+0x276c0) (BuildId: ca0db5ab57a36507d61bbcf4988d344974331f19)
    #11 0x7f2bba8277f8 in __libc_start_main (/usr/lib/libc.so.6+0x277f8) (BuildId: ca0db5ab57a36507d61bbcf4988d344974331f19)
    #12 0x5631f51be594 in _start (/home/amaury/work/haproxy-quic-dev/haproxy+0x2a9594) (BuildId: a7ccfd74b7a71a869b8ff8d13f6dcde8c82c1487)

No need to backport.