fix indirect jmp resolve indirect twice.

Repost from #3, `* Indirect jump (etc. "jmp (addr)")`

> It looks like you're making indirect jump behave   exactly the same as direct jump? And I think I have the behavior correct for indirect,y addressing.

It's seems resolving indirect access **twice**, in `decode_arg()` and `jmp()`.
so `jmp (arg)` must jump to `*arg`, but jump to `**arg`.

I'm not sure where I should resolve indirect access in `decode_arg()` and `jump()`.
How do you think? 

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example code

    lda #$34
    sta $1000
    lda #$12
    sta $1001
    jmp ($1000)

must jump to $1234, but $0000  ( == `*($1234)` )