zebra: use "assert" instead of unnecessary check

zebra: use "assert" instead of unnecessary check

Since `zvni_map_to_svi_ns()` is used to find and return one specific interface
based on passed attributes of SVI, so the two parameters `in_param` and `p_ifp`
must not be NULL.

Passing NULL `p_ifp` makes no sense, so the check `if (p_ifp)` is
unnecessary.

So use `assert` to ensure the two parameters, and remove that unnecessary check.

Signed-off-by: anlan_cs <vic.lan@pica8.com>