text/template: fix bug in pipelined variadics
Simple bug in argument processing: The final arg may
be the pipeline value, in which case it gets bound to the
fixed argument section. The code got that wrong. Easy
to fix.

Fixes issue 8950.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/161750043