Check that ifma_protospec != NULL in inm_lookup

If ifma_protospec is NULL when inm_lookup() is called, there
is a dereference in a NULL struct pointer. This ensures that struct is
not NULL before comparing the address.

Reported by: dumbbell
Reviewed by: sbruno
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D15440