Fix interface leaking ipv4 field.

This is triggered when flushing an interface.

    4 bytes in 1 blocks are definitely lost in loss record 1 of 6
       at 0x483A77F: malloc (vg_replace_malloc.c:307)
       by 0x11155E: check_interface_ipv4 (interface.c:161)
       by 0x110FCD: interface_updown (interface.c:460)
       by 0x1116E2: check_interfaces (interface.c:550)
       by 0x10AFAA: main (babeld.c:546)

interface.c

@@ -122,6 +122,7 @@ flush_interface(char *ifname)
 
     local_notify_interface(ifp, LOCAL_FLUSH);
 
+    free(ifp->ipv4);
     free(ifp);
 
     return 1;