text/template: error on method calls on nil interfaces
Trying to call a method on a nil interface is a panic in Go. For
example:
var stringer fmt.Stringer
println(stringer.String()) // nil pointer dereference
In https://golang.org/cl/143097 we started recovering panics encountered
during function and method calls. However, we didn't handle this case,
as text/template panics before evalCall is ever run.
In particular, reflect's MethodByName will panic if the receiver is of
interface kind and nil:
panic: reflect: Method on nil interface value
Simply add a check for that edge case, and have Template.Execute return
a helpful error. Note that Execute shouldn't just error if the interface
contains a typed nil, since we're able to find a method to call in that
case.
Finally, add regression tests for both the nil and typed nil interface
cases.