zebra: Rework code to make SA happy

zebra: Rework code to make SA happy

Clan SA was saying:

./zebra/zebra_vty_clippy.c: In function ‘show_route’:
zebra/zebra_vty.c:1775:4: warning: ‘zvrf’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    do_show_ip_route_all(vty, zvrf, afi, !!fib, !!json, tag,
    ^

I do not see a way that zvrf could ever be uninited in the code path
but rearrange the code a tiny bit to make it happier.

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