html/template: use named consts instead of their values

Use defined named constants instead of 0 literal in comparisons.

Found using https://go-critic.github.io/overview.html#namedConst-ref

Change-Id: Ic075cece248f6e51db0b3d9d9eaba7d6409c9eef
Reviewed-on: https://go-review.googlesource.com/123376


Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>