Allow _h_indexbuild() to be interrupted.

Allow _h_indexbuild() to be interrupted.

When we are building a hash index that is large enough to need
pre-sorting (larger than either maintenance_work_mem or NBuffers),
the initial sorting phase is interruptible, but the insertion
phase wasn't.  Add the missing CHECK_FOR_INTERRUPTS().

Per bug #18616 from Alexander Lakhin.  Back-patch to all
supported branches.

Pavel Borisov

Discussion: https://postgr.es/m/18616-acbb9e5caf41e964@postgresql.org