zebra: Fixup crash with vlan interfaces attempted to be used

zebra: Fixup crash with vlan interfaces attempted to be used

When zebra starts up it receives from the kernel a full dump of
interface information.  Unfortunately it is in no particular order.
As such we sometimes receive data from the kernel about interfaces
we do not know about yet.

In this bug, we are attempting to use the interface pointer(->link)
for a vlan interface that we have not properly resolved.

This fix ensures that we will not attempt to call zvni_map_svi
if we have a NULL pointer.  There are other places in the code
we are already checking for the fact that the ->link pointer
is valid before calling this function, so I believe that this
is correct.

We do need to come back and resolve all ->link pointers
after we have received the full table.  This can be
done in another commit.

Ticket: CM-17041
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>