net: fix trace when debug is activated in slirp

make the code compile correctly when DEBUG is activated.
Signed-off-by: NVincent Palatin <vpalatin@chromium.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

slirp/bootp.c

@@ -284,7 +284,7 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
     } else {
         static const char nak_msg[] = "requested address not available";
 
-        DPRINTF("nak'ed addr=%08x\n", ntohl(preq_addr->s_addr));
+        DPRINTF("nak'ed addr=%08x\n", ntohl(preq_addr.s_addr));
 
         *q++ = RFC2132_MSG_TYPE;
         *q++ = 1;