acl-plugin: don't attempt to delete the ACLs on interface deletion if ACL plugin...

acl-plugin: don't attempt to delete the ACLs on interface deletion if ACL plugin heap is not initialized

With the addition of the own heap, the delete routines called from interface deletion
callback may attempt to initialize the ACL plugin heap. This is obviously not
a desirable condition - so, return early from the callback if the ACL plugin
heap has not been initialized yet - there is for sure nothing to clean up.

Change-Id: I08a6ae725294016ff5824189ade91c288e2c473b
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>