slirp: fix CVE 2007-5729

The emulated network cards in QEMU allows local users to execute arbitrary
code by writing Ethernet frames with a size larger than the slirp's default
MTU, which triggers a heap-based buffer overflow in the slirp library.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5920 c046a42c-6fe2-441c-8c8c-71466251a162
