Catch some malformed pipelines at parsing time.
The current code accepts pipelines such as:

{{12|.}}
{{"hello"|print|false}}
{{.|"blah blah"}}

Such pipelines generate panic in html/template at execution time.

Add an extra check to verify all the commands of the pipeline are executable
(except for the first one).

Fixes #10610

Change-Id: Id72236ba8f76a59fa284fe3d4c2cb073e50b51f1
Reviewed-on: https://go-review.googlesource.com/9626Reviewed-by: Rob Pike <r@golang.org>