Avoid crash if -redir or -smb used without -net user.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@1760 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/vl.c b/vl.c
index 97eddbf..8d1fa37 100644
--- a/vl.c
+++ b/vl.c
@@ -1926,7 +1926,7 @@
 
 int slirp_can_output(void)
 {
-    return qemu_can_send_packet(slirp_vc);
+    return !slirp_vc || qemu_can_send_packet(slirp_vc);
 }
 
 void slirp_output(const uint8_t *pkt, int pkt_len)
@@ -1935,6 +1935,8 @@
     printf("slirp output:\n");
     hex_dump(stdout, pkt, pkt_len);
 #endif
+    if (!slirp_vc)
+        return;
     qemu_send_packet(slirp_vc, pkt, pkt_len);
 }