Fix HostCallback::is_effect_valid()
Fix HostCallback::is_effect_valid()

`is_effect_valid()` was turning `*mut AEffect` itself into an `i32` and
comparing it with `VST_MAGIC`. What needs to happen instead is you need
to fetch `magic` out of the `AEffect` struct and compare *that* with
`VST_MAGIC`.

rust-dsp/rust-vst,rust-dsp/rust-vst