zebra: fix assert mpls when terminating zebra

zebra: fix assert mpls when terminating zebra

The assert appears in zebra_mpls.c when checking default zebra_vrf.
It appears that when the mpls entries are flushed, it gets the default
vrf which is already flushed by vrf_terminate() function. In order to
avoid that assert to trigger a crash, the mpls flush is called before
vrf termination.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>