Do a CHECK_FOR_INTERRUPTS immediately before terminating a fastpath

Do a CHECK_FOR_INTERRUPTS immediately before terminating a fastpath
function call.  Previously, there may have been no CHECK_FOR_INTERRUPTS
at all in the fastpath code path, making it impossible to cancel an
operation such as \lo_import externally.  This addition doesn't ensure
you can cancel, since your SIGINT may arrive while the backend is idle
waiting for the client, but it gives the largest window we can easily
provide.  Noted while experimenting with new control-C code for psql.