cmd/internal/obj/s390x: don't crash on invalid instruction

I didn't bother with a test as there doesn't seem to be an existing
framework for testing assembler failures, and tests for invalid code
aren't all that interesting.

Fixes #26700

Change-Id: I719410d83527802a09b9d38625954fdb36a3c0f7
Reviewed-on: https://go-review.googlesource.com/c/153177


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>