evaluate: error out when expression has no datatype

evaluate: error out when expression has no datatype

add rule ip6 f i rt2 addr . ip6 daddr  { dead:: . dead:: }

... will cause a segmentation fault, we assume expr->dtype is always
set.

rt2 support is incomplete, the template is uninitialised.

This could be fixed up, but rt2 (a subset of the deperecated type 0),
like all other routing headers, lacks correct dependency tracking.

Currently such routing headers are always assumed to be segment routing
headers, we would need to add dependency on 'Routing Type' field in the
routing header, similar to icmp type/code.

Signed-off-by: Florian Westphal <fw@strlen.de>