slirp: update to latest stable-4.2 branch

Dr. David Alan Gilbert (1):
      ip_stripoptions use memmove

Jindrich Novy (4):
      Fix possible infinite loops and use-after-free
      Use secure string copy to avoid overflow
      Be sure to initialize sockaddr structure
      Check lseek() for failure

Marc-André Lureau (2):
      util: do not silently truncate
      Merge branch 'stable-4.2' into 'stable-4.2'

Philippe Mathieu-Daudé (3):
      Fix win32 builds by using the SLIRP_PACKED definition
      Fix constness warnings
      Remove unnecessary break

Ralf Haferkamp (2):
      Drop bogus IPv6 messages
      Fix MTU check

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>