IPV6 Fix a crash when trying to replace non existing route

This patch fixes a crash when non existing IPv6 route is tried to be changed.

When new destination node was inserted in middle of FIB6 tree, no relevant
sanity checks were performed. Later route insertion might have been prevented
due to invalid request, causing node with no rt info being left in tree.
When this node was accessed, a crash occurred.

Patch adds missing checks in fib6_add_1()

Signed-off-by: Matti Vaittinen <Mazziesaccount@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>