evaluate: tunnel: don't assume src is set

evaluate: tunnel: don't assume src is set

Included bogon crashes, after fix:

empty_geneve_definition_crash:2:9-16: Error: Could not process rule: Invalid argument

Since this feature is undocumented (hint, hint) I don't know
if there are cases where ip daddr can be elided.

If not, a followup patch should reject empty dst upfront
so users get a more verbose error message.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>