vxlan: Avoid creating fdb entry with NULL destination

Commit afbd8bae
   vxlan: add implicit fdb entry for default destination
creates an implicit fdb entry for default destination. This results
in an invalid fdb entry if default destination is not specified.
For ex:
  ip link add vxlan1 type vxlan id 100
creates the following fdb entry
  00:00:00:00:00:00 dev vxlan1 dst 0.0.0.0 self permanent

This patch fixes this issue by creating an fdb entry only if a
valid default destination is specified.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>