ripngd: fix valgrind warning about uninitialized memory usage

ripngd: fix valgrind warning about uninitialized memory usage

Fixes the following warning when running ripngd with valgrind:
==38== Syscall param sendmsg(msg.msg_control) points to uninitialised
byte(s)
==38==    at 0x5EA1E47: sendmsg (sendmsg.c:28)
==38==    by 0x118C48: ripng_send_packet (ripngd.c:226)
==38==    by 0x11D1D6: ripng_request (ripngd.c:1924)
==38==    by 0x120BD8: ripng_interface_wakeup (ripng_interface.c:666)
==38==    by 0x4ECB4B4: thread_call (thread.c:1601)
==38==    by 0x4E8D9CE: frr_run (libfrr.c:1011)
==38==    by 0x1121C8: main (ripng_main.c:180)
==38==  Address 0xffefffc34 is on thread 1's stack
==38==  in frame #1, created by ripng_send_packet (ripngd.c:172)

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>