x86/x86asm: fix panic on truncated VEX/EVEX prefix

decodeAVX guards every read from src except the opcode byte itself, so a
stream ending exactly at the end of a VEX or EVEX prefix indexes out of
range instead of reporting a truncated instruction:

	c5 fc         panic: index out of range [2] with length 2
	c4 e2 7d      panic: index out of range [3] with length 3
	62 f1 7c 48   panic: index out of range [4] with length 4

Add the missing bounds check and extend TestDecodeDoesNotCrash with the
three prefix forms.

Fixes golang/go#81447.

Change-Id: Id68526e432646ea95d2c1b84d715444ad2d4fb9a
GitHub-Last-Rev: 2a9c2bcbcd58bf223e12b76e9821baf50829dff8
GitHub-Pull-Request: golang/arch#25
Reviewed-on: https://go-review.googlesource.com/c/arch/+/830644
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>