Fix using ptr to seq in js backend.
Fix using ptr to seq in js backend.

I suddenly got ReferenceError exceptions for some of the code which is
using etyBaseIndex ptr. Initialising the xxx_Idx part of the variable
fixes this problem.

jfhg/Nim,russpowers/Nim,mbaulch/Nim,tulayang/Nim,jfhg/Nim,dom96/Nim,dom96/Nim,singularperturbation/Nim,mbaulch/Nim,dom96/Nim,tulayang/Nim,jfhg/Nim,jfhg/Nim,dom96/Nim,mbaulch/Nim,haiodo/Nim,tulayang/Nim,singularperturbation/Nim,dom96/Nim,mbaulch/Nim,singularperturbation/Nim,russpowers/Nim,russpowers/Nim,singularperturbation/Nim,russpowers/Nim,haiodo/Nim,tulayang/Nim,jfhg/Nim,tulayang/Nim,dom96/Nim,russpowers/Nim,dom96/Nim,jfhg/Nim,haiodo/Nim,mbaulch/Nim,mbaulch/Nim,haiodo/Nim,singularperturbation/Nim,singularperturbation/Nim,russpowers/Nim,russpowers/Nim,singularperturbation/Nim,mbaulch/Nim,haiodo/Nim,tulayang/Nim,haiodo/Nim,jfhg/Nim,haiodo/Nim