set_deparse_plan: Reuse variable to appease Coverity

set_deparse_plan: Reuse variable to appease Coverity

Coverity complains that dpns->outer_plan is deferenced (to obtain
->targetlist) when possibly NULL.  We can avoid this by using
dpns->outer_tlist instead, which was already obtained a few lines up.

The fact that we end up with
  dpns->inner_tlist = dpns->outer_tlist
is a bit suspicious-looking and maybe worthy of more investigation, but
I'll leave that for another day.

Reviewed-by: Michaël Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/202204191345.qerjy3kxi3eb@alvherre.pgsql