BUG/MINOR: rules: Initialize the list element when allocating a new rule

BUG/MINOR: rules: Initialize the list element when allocating a new rule

dd7e6c6dc ("BUG/MINOR: http-rules: completely free incorrect TCP rules on
error") and 388c0f2a6 ("BUG/MINOR: tcp-rules: completely free incorrect TCP
rules on error") introduced a regression because the list element of a new
rule is not intialized. Thus HAProxy crashes when an incorrect rule is
released.

This patch must be backported if above commits are backported. Note that
new_act_rule() only exists since the 2.5. It relies on the commit d535f807b
("MINOR: rules: add a new function new_act_rule() to allocate act_rules").