bfdd: Prevent unininited data transmittal

bfdd: Prevent unininited data transmittal

Valgrind reports:

2052866-==2052866==
2052866-==2052866== Syscall param sendmsg(msg.msg_name) points to uninitialised byte(s)
2052866:==2052866==    at 0x49C8E13: sendmsg (sendmsg.c:28)
2052866-==2052866==    by 0x11DC08: bp_udp_send (bfd_packet.c:823)
2052866-==2052866==    by 0x11DD76: ptm_bfd_echo_snd (bfd_packet.c:179)
2052866-==2052866==    by 0x114C2D: ptm_bfd_echo_xmt_TO (bfd.c:469)
2052866-==2052866==    by 0x114C2D: ptm_bfd_echo_start (bfd.c:498)
2052866-==2052866==    by 0x114C2D: bs_echo_timer_handler (bfd.c:1199)
2052866-==2052866==    by 0x11E478: bfd_recv_cb (bfd_packet.c:702)
2052866-==2052866==    by 0x4904846: thread_call (thread.c:1681)
2052866-==2052866==    by 0x48CB4DF: frr_run (libfrr.c:1126)
2052866-==2052866==    by 0x113044: main (bfdd.c:403)
2052866-==2052866==  Address 0x1ffefff3e8 is on thread 1's stack

In ptm_bfd_echo_snd, for the v4 case we were memsetting the v6 memory
then setting the v4 memory.  Just fix it.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>