misc: fix coverity warnings

Add an ALWAYS_ASSERT (...) macro, to (a) shut up coverity, and (b)
check the indicated condition in production images.

As in:
 p = hash_get(...);
 ALWAYS_ASSERT(p) /* was ASSERT(p) */
 elt = pool_elt_at_index(pool, p[0]);

This may not be the best way to handle a specific case, but failure to
check return values at all followed by e.g. a pointer dereference
isn't ok.

Type: fix
Ticket: VPP-1837
Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Ia97c641cefcfb7ea7d77ea5a55ed4afea0345acb