html/template: fix pipeline sanitization

Pipelines are altered by inserting sanitizers if they are not
already present. The code makes the assumption that the first
operands of each commands are function identifiers.

This is wrong, since they can also be methods. It results in
a panic with templates such as {{1|print 2|.f 3}}

Adds an extra type assertion to make sure only identifiers
are compared with sanitizers.

Fixes #10673

Change-Id: I3eb820982675231dbfa970f197abc5ef335ce86b
Reviewed-on: https://go-review.googlesource.com/9801


Reviewed-by: Rob Pike <r@golang.org>