Fix CVE-2017-11434:

https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg05001.html
https://bugzilla.redhat.com/show_bug.cgi?id=1472611
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-11434
https://security-tracker.debian.org/tracker/CVE-2017-11434

Patch copied from upstream source repository:

https://git.qemu.org/gitweb.cgi?p=qemu.git;a=commit;h=413d463f43fbc4dd3a601e80a5724aa384a265a0

diff --git a/slirp/bootp.c b/slirp/bootp.c
index 5a4646c182..5dd1a415b5 100644
--- a/slirp/bootp.c
+++ b/slirp/bootp.c
@@ -123,6 +123,9 @@ static void dhcp_decode(const struct bootp_t *bp, int *pmsg_type,
             if (p >= p_end)
                 break;
             len = *p++;
+            if (p + len > p_end) {
+                break;
+            }
             DPRINTF("dhcp: tag=%d len=%d\n", tag, len);
 
             switch(tag) {