encoding/json: fix handling of nil anonymous structs

Given the following types:

	type S2 struct{ Field string }
	type S  struct{ *S2 }

Marshalling a value of type T1 should result in "{}", as there's no way
to access any value of T2.Field. This is how Go 1.10 and earlier
versions behave.

However, in the recent refactor golang.org/cl/125417 I broke this logic.
When the encoder found an anonymous struct pointer field that was nil,
it no longer skipped the embedded fields underneath it. This can be seen
in the added test:

	--- FAIL: TestAnonymousFields/EmbeddedFieldBehindNilPointer (0.00s)
	    encode_test.go:430: Marshal() = "{\"Field\":\"\\u003c*json.S2 Value\\u003e\"}", want "{}"

The human error was a misplaced label, meaning we weren't actually
skipping the right loop iteration. Fix that.

Change-Id: Iba8a4a77d358dac73dcba4018498fe4f81afa263
Reviewed-on: https://go-review.googlesource.com/131376


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>