function checkGetComponents(comps, outLen, vec) {
    if (comps.length > outLen)
        throw new Error("too many components");
    checkLegalComponents(comps, vec);
}
performs all validity checks of [[checkLegalComponents]] and checks if the
number of accessed components does not exceed the size of the vector being
assigned to
@param comps components string
@param outLen length of the resultant vector
@param vec vector being accessed