text/template: don't panic when function call evaluates a nil pointer

text/template: don't panic when function call evaluates a nil pointer
Catch the error instead and return it to the user. Before this fix,
the template package panicked. Now you get:
        template: bug11:1:14: executing "bug11" at <.PS>: dereference of nil pointer of type *string
Extended example at http://play.golang.org/p/uP6pCW3qKT

Fixes #7333.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/64150043