net: fix trace when debug is activated in slirp

make the code compile correctly when DEBUG is activated.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/bootp.c b/bootp.c
index 11604c0..4885446 100644
--- a/bootp.c
+++ b/bootp.c
@@ -289,7 +289,7 @@
     } 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;