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 <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>