policy: Fix use after free

policy: Fix use after free

The instance of E_Policy_Client has to be freed even though the member
variable `zone` is null.

The zone of E_Policy_Client instance may be NULL until it receives ecore
event, E_EVENT_CLIENT_ZONE_SET. If the callback of E_CLIENT_HOOK_DEL is
called before E_EVENT_CLIENT_ZONE_SET, the callback would return doing
nothing and the `pc` would be remained in hash table, and this will
cause use-after-free.

Change-Id: I14cde4159a2e0bf108d5dacd9897744459a129f7