fix use after free in engine.Disasm

Hi Ben,

I was using a debug build of capstone+gapstone which poisons bytes when they're freed, and it revealed a subtle use-after-free issue in engine.Disasm.
Because of the defer cs_free of insn, the elements pointed by the returned slice will be freed when the function returns, so when the caller tries to access the disassembly, it points to freed memory.
It was revealed by https://github.com/bnagy/gapstone/blob/master/x86_decomposer_test.go#L20
which in a normal build and test run shows x86.SPEC:
        Prefix:0x00 0x00 0x00 0x00
        Opcode:0x8d 0x00 0x00 0x00
but with a debug build poisoning bytes when it's freed with 0xcd, it shows:
        Prefix:0xcd 0xcd 0xcd 0xcd
        Opcode:0xcd 0xcd 0xcd 0xcd

Attached is a patch that removes the cs_free I'm talking about, but don't merge it: it creates a memory leak, I checked with a program doing many engine.Disasm.
So I'm not sure how you want to fix this: we could provide a method for callers to free this, but that's not elegant; it could be better to copy this slice backed by C elements into a Go slice that would then be managed by the garbage collector.

Thoughts?

Cheers