text/template: protect against explicit nil in field chains

text/template: protect against explicit nil in field chains

An explicit nil in an expression like nil.Foo caused a panic
because the evaluator attempted to reflect on the nil.
A typeless nil like this cannot be used to do anything, so
just error out.

Fixes #9426

Change-Id: Icd2c9c7533dda742748bf161eced163991a12f54
Reviewed-on: https://go-review.googlesource.com/7643
Reviewed-by: David Symonds <dsymonds@golang.org>