Fix SIGSEGV crash in slirp networking code

Fix SIGSEGV crash in networking code (bug was introduced in r6288).
Thanks to Gleb Natapov for finding this fix.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6545 c046a42c-6fe2-441c-8c8c-71466251a162
