zebra: Prevent sending of unininted data

zebra: Prevent sending of unininted data

valgrind is reporting:
2448137-==2448137== Thread 5 zebra_apic:
2448137-==2448137== Syscall param writev(vector[...]) points to uninitialised byte(s)
2448137:==2448137==    at 0x4D6FDDD: __writev (writev.c:26)
2448137-==2448137==    by 0x4D6FDDD: writev (writev.c:24)
2448137-==2448137==    by 0x48A35F5: buffer_flush_available (buffer.c:431)
2448137-==2448137==    by 0x48A3504: buffer_flush_all (buffer.c:237)
2448137-==2448137==    by 0x495948: zserv_write (zserv.c:263)
2448137-==2448137==    by 0x4904B7E: thread_call (thread.c:1681)
2448137-==2448137==    by 0x48BD3E5: fpt_run (frr_pthread.c:308)
2448137-==2448137==    by 0x4C61EA6: start_thread (pthread_create.c:477)
2448137-==2448137==    by 0x4D78DEE: clone (clone.S:95)
2448137-==2448137==  Address 0x720c3ce is 62 bytes inside a block of size 4,120 alloc'd
2448137:==2448137==    at 0x483877F: malloc (vg_replace_malloc.c:307)
2448137-==2448137==    by 0x48D2977: qmalloc (memory.c:110)
2448137-==2448137==    by 0x48A30E3: buffer_add (buffer.c:135)
2448137-==2448137==    by 0x48A30E3: buffer_put (buffer.c:161)
2448137-==2448137==    by 0x49591B: zserv_write (zserv.c:256)
2448137-==2448137==    by 0x4904B7E: thread_call (thread.c:1681)
2448137-==2448137==    by 0x48BD3E5: fpt_run (frr_pthread.c:308)
2448137-==2448137==    by 0x4C61EA6: start_thread (pthread_create.c:477)
2448137-==2448137==    by 0x4D78DEE: clone (clone.S:95)
2448137-==2448137==  Uninitialised value was created by a stack allocation
2448137:==2448137==    at 0x43E490: zserv_encode_vrf (zapi_msg.c:103)

Effectively we are sending `struct vrf_data` without ensuring
data has been properly initialized.

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