acl-plugin: fix coverity error that the fix related for [VPP-1502] has triggered

Fix the trivial use-before-check copypaste error.

There was a more subtle issue with that patch that Coverity didn't notice:
namely, vec_validate(v, len-1) is a terrible idea if len happens to be == 0.
Fix that.

Change-Id: I0fab8b1750e9e9973eefb5d39f35e4c3a13fc66f
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>