m_cleanup: set qh_link and qh_rlink to the list head

This should ensure the lists are considered empty after the cleanup
diff --git a/src/mbuf.c b/src/mbuf.c
index 32d3210..9da7966 100644
--- a/src/mbuf.c
+++ b/src/mbuf.c
@@ -42,6 +42,8 @@
         g_free(m);
         m = next;
     }
+    list_head->qh_link = list_head;
+    list_head->qh_rlink = list_head;
 }
 
 void m_cleanup(Slirp *slirp)