runtime: have ReadMemStats do a nil check before switching stacks

runtime: have ReadMemStats do a nil check before switching stacks

This gives the user a better stack trace experience. No need to
expose them to runtime.systemstack and friends.

Fixes #61158

Change-Id: I4f423f82e54b062773067c0ae64622e37cb3948b
Reviewed-on: https://go-review.googlesource.com/c/go/+/507755
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>