text/template: perform value validity checks

text/template: perform value validity checks

Check reflect.Value.IsValid() before calling other reflect.Value methods
that panic on zero values.

Added tests for cases with untyped nils. They panicked without these fixes.

Removed a TODO.

Fixes #12356

Change-Id: I9b5cbed26db09a0a7c36d99a93f8b9729899d51e
Reviewed-on: https://go-review.googlesource.com/14340
Reviewed-by: Rob Pike <r@golang.org>