JIT plugin issue

This cause the JIT plugin to crash, even if the code is not run:
https://github.com/Zeex/amx_assembly/blob/4d1955069c34ca297c376b770d7dbd4ecdf38e92/opcode.inc#L589

I solved it by changing it into this:

``` pawn
case OP_SWITCH: {
    new x = 0;
    switch (x) { case 0: return ReadOpcodeNearThis(-36); }
}
```

However, it only works when compiling with `O1` so obviously it's not a good solution.
