slirp: tftp, copy sockaddr_size

ASAN detects an "unknown-crash" when running pxe-test:

/ppc64/pxe/spapr-vlan: =================================================================
==7143==ERROR: AddressSanitizer: unknown-crash on address 0x7f6dcd298d30 at pc 0x55e22218830d bp 0x7f6dcd2989e0 sp 0x7f6dcd2989d0
READ of size 128 at 0x7f6dcd298d30 thread T2
    #0 0x55e22218830c in tftp_session_allocate /home/elmarco/src/qq/slirp/tftp.c:73
    #1 0x55e22218a1f8 in tftp_handle_rrq /home/elmarco/src/qq/slirp/tftp.c:289
    #2 0x55e22218b54c in tftp_input /home/elmarco/src/qq/slirp/tftp.c:446
    #3 0x55e2221833fe in udp6_input /home/elmarco/src/qq/slirp/udp6.c:82
    #4 0x55e222137b17 in ip6_input /home/elmarco/src/qq/slirp/ip6_input.c:67

Address 0x7f6dcd298d30 is located in stack of thread T2 at offset 96 in frame
    #0 0x55e222182420 in udp6_input /home/elmarco/src/qq/slirp/udp6.c:13

  This frame has 3 object(s):
    [32, 48) '<unknown>'
    [96, 124) 'lhost' <== Memory access at offset 96 partially overflows this variable
    [160, 200) 'save_ip' <== Memory access at offset 96 partially underflows this variable

The sockaddr_storage pointer is the sockaddr_in6 lhost on the
stack. Copy only the source addr size.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
