zebra: Fix usage of accidental NULL pointer

zebra: Fix usage of accidental NULL pointer

NS_DEFAULT is #defined to 0, We are passing it
in to a function that is taking 'struct zebra_ns *'
which is translating into a NULL pointer.  Which
in some situations will cause a crash.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>
             Daniel Walton <dwalton@cumulusnetworks.com>
             Nikolay Aleksandrov <nikolay@cumulusnetworks.com>

(cherry picked from commit 1e9fa2763953adc603c3acc4ed2a46c9e72cbb29)