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

[nsavoire]

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.

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