Found by coverity.
Introduced by c0e7d54e37 "Removed 2 mem_mallocs: error string can be a
global variable, include memory for sc_ac_cookie in struct pppoe_softc;
commented out unused code (sc_service_name/sc_concentrator_name)".
Fixes it by bailing out if received AC cookie is to big for us, this
can't really happen anyway.