Suppress warnings for register-groups-bind with empty var-list

If (for some reason) the var-list is empty, there is no code generated
that uses the reg-starts and reg-ends variables, and hence they become
unused.

And without the bindings, the substr-fn is also not necessary.
Merge pull request #7 from jdz/patch-1

Suppress warnings for register-groups-bind with empty var-list