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>

src/runtime/symtab.go

@@ -579,6 +579,9 @@ func FuncForPC(pc uintptr) *Func {
 
 // Name returns the name of the function.
 func (f *Func) Name() string {
+	if f == nil {
+		return ""
+	}
 	return funcname(f.funcInfo())
 }
 

src/runtime/symtab_test.go

@@ -154,3 +154,14 @@ func TestLineNumber(t *testing.T) {
 		}
 	}
 }
+
+func TestNilName(t *testing.T) {
+	defer func() {
+		if ex := recover(); ex != nil {
+			t.Fatalf("expected no nil panic, got=%v", ex)
+		}
+	}()
+	if got := (*runtime.Func)(nil).Name(); got != "" {
+		t.Errorf("Name() = %q, want %q", got, "")
+	}
+}