runtime: allow calling Func.Name on nil pointer

runtime: allow calling Func.Name on nil pointer

The Func type has allowed calling the Func.Name method on a nil pointer
since Go1.2, where it returned an empty string. A regression caused by
CL/37331 caused this behavior to change. This breaks code that lazily
does runtime.FuncForPC(myPtr).Name() without first checking that myPtr
is actually non-nil.

Fixes #20872

Change-Id: Iae9a2ebabca5e9d1f5a2cdaf2f30e9c6198fec4f
Reviewed-on: https://go-review.googlesource.com/47354
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>