read_prefixed_opcode() can fail; we must check for that case
before using the value it returned.

Bug: chromium:1073356
Change-Id: Ic213bd296e9c50dbffd5df8bc94ef0e7695d75a2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2161068
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67314}