rule: skip CMD_OBJ_SETELEMS with no elements after set flush

rule: skip CMD_OBJ_SETELEMS with no elements after set flush

Set declaration + set flush results in a crash because CMD_OBJ_SETELEMS
does not expect no elements. This internal command only shows up if set
contains elements, however, evaluation flushes set content after the set
expansion. Skip this command CMD_OBJ_SETELEMS if set is empty.

Fixes: d3c8051cb767 ("rule: rework CMD_OBJ_SETELEMS logic")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>