[release-branch.go1] text/template: pipelined arg was not typechecked

[release-branch.go1] text/template: pipelined arg was not typechecked

««« backport c007925a75c6
text/template: pipelined arg was not typechecked
Without this fix, an erroneous template causes a panic; should be caught safely.
The bug did not affect correct templates.

Fixes #3267.

R=golang-dev, dsymonds, rsc
CC=golang-dev
https://golang.org/cl/5900065
»»»