evaluate: do not fetch next expression on runaway number of concatenation components

evaluate: do not fetch next expression on runaway number of concatenation components

If this is the last expression, then the runaway flag is set on and
evaluation bails in the next iteration, do not fetch next list element
which refers to the list head.

I found this by code inspection, I could not trigger any crash with this
one.

Fixes: ae1d54d1343f ("evaluate: do not crash on runaway number of concatenation components")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>