This has been the root cause of a number of crashes caused by
fuzz throwing modem noise at the assembler, which in turn attempts
to print diagnostics but instead just gets crashes.

Fixes #12627.

Change-Id: I72c2da79d8eb240e1a37aa6140454c552b05e0f1
Reviewed-on: https://go-review.googlesource.com/14595Reviewed-by: Ian Lance Taylor <iant@golang.org>