MINOR: actions: add new function free_act_rule() to free a single rule

MINOR: actions: add new function free_act_rule() to free a single rule

There was free_act_rules() that frees all rules from a head but nothing
to free a single rule. Currently some rulesets partially free their own
rules on parsing error, and we're seeing some regtests emit errors under
ASAN because of this.

Let's first extract the code to free a rule into its own function so
that it becomes possible to use it on a single rule.