runtime: fix dumpgoroutine() to deal with open-coded defers

_defer.fn can be nil, so we need to add a check when dumping
_defer.fn.fn.

Fixes #35172

Change-Id: Ic1138be5ec9dce915a87467cfa51ff83acc6e3a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/203697


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>