cmd/internal/obj: stop plist flushing early on error
cmd/internal/obj: stop plist flushing early on error

If preprocessing or assembling has failed, we should not proceed.
First, there's no point.
Second, I will shortly add some sanity checks to linkpcln
that will fail on malformed input.

Change-Id: I055eeab1c2f3a66b4b2cadb551bbf4ab55d176f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/171886
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

josharian/go,cloudflare/go,golang/go,golang/go,akutz/go,josharian/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,josharian/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,golang/go,akutz/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,josharian/go,nawawi/go,akutz/go,golang/go,nawawi/go