Fix JITed EEOP_AGG_INIT_TRANS, which missed some state.

Fix JITed EEOP_AGG_INIT_TRANS, which missed some state.

The JIT compiled implementation missed maintaining
AggState->{current_set,curaggcontext}. That could lead to trouble
because the transition value could be allocated in the wrong context.

Reported-By: Rushabh Lathia
Diagnosed-By: Dmitry Dolgov
Author: Dmitry Dolgov, with minor changes by me
Discussion: https://postgr.es/m/CAGPqQf165-=+Drw3Voim7M5EjHT1zwPF9BQRjLFQzCzYnNZEiQ@mail.gmail.com
Backpatch: 11-, where JIT compilation support was added