selinux: fix return value on error in policydb_read()

The value of rc is still zero from the last assignment when the error
path is taken. Fix it by setting it to -ENOMEM before the
hashtab_create() call.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e67b2ec9 ("selinux: store role transitions in a hash table")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>