[release-branch.go1.16] text/template: initialize template before locking it

For #39807
For #48436
Fixes #48443

Change-Id: I75f82fd8738dd2f11f0c69b1230e1be1abc36024
Reviewed-on: https://go-review.googlesource.com/c/go/+/350730


Trust: Ian Lance Taylor <iant@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
(cherry picked from commit ba1c52d7)
Reviewed-on: https://go-review.googlesource.com/c/go/+/351116