runtime: skip pointless writes in freedefer

Change-Id: I501a0e5c87ec88616c7dcdf1b723758b6df6c088
Reviewed-on: https://go-review.googlesource.com/98758
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/panic.go

@@ -280,8 +280,9 @@ func freedefer(d *_defer) {
 	d.started = false
 	d.sp = 0
 	d.pc = 0
-	d.fn = nil
-	d._panic = nil
+	// d._panic and d.fn must be nil already.
+	// If not, we would have called freedeferpanic or freedeferfn above,
+	// both of which throw.
 	d.link = nil
 
 	pp.deferpool[sc] = append(pp.deferpool[sc], d)