Fix broken cleanup interlock for GIN pending list.

The pending list must (for correctness) always be cleaned up by vacuum, and
should (for the avoidance of surprising behavior) always be cleaned up
by an explicit call to gin_clean_pending_list, but cleanup is optional
when inserting.  The old logic got this backward: cleanup was forced
if (stats == NULL), but that's going to be *false* when vacuuming and
*true* for inserts.

Masahiko Sawada, reviewed by me.

Discussion: http://postgr.es/m/CAD21AoBLUSyiYKnTYtSAbC+F=XDjiaBrOUEGK+zUXdQ8owfPKw@mail.gmail.com