apparmor: Fix memory leak of rule on error exit path

Currently on the error exit path the allocated rule is not free'd
causing a memory leak. Fix this by calling aa_audit_rule_free().

Detected by CoverityScan, CID#1468966 ("Resource leaks")

Fixes: cb740f574c7b ("apparmor: modify audit rule support to support profile stacks")
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>