Pass function pointer instead of &parameter

[pwaller]

Before this commit, a pointer to the stack was passed as the function
pointer. If you passed nil as a function pointer, for example, a
pointer to the stack was being passed.

This fix is not thorough and I don't actually want to call a CB, so I
have not checked that it works for that use case. I have not dived in to
discover if there is a better way to make this work, I could use a hand
here improving this fix.

I note that there exists a function pointer type (CB below), which has
a PassRef/PassValue methods. Maybe those should be called?

Before:

func F(cb CB) {
	cb, _ := (*[0]byte)(unsafe.Pointer(&cb)), cgoAllocsUnknown
	C.cfunc(cb)
}

After:

func F(cb CB) {
	cb, _ := *(**[0]byte)(unsafe.Pointer(&cb)), cgoAllocsUnknown
	C.cfunc(cb)
}

[pwaller]

It is quite possible this fixes my use case and breaks other use cases, so I'm quite happy if this PR is not merged. I'm not certain if this functionality is in use anywhere - I haven't got time to look very hard at the minute.

[pwaller]

Ping.

[xlab]

@pwaller
I have no ideas at this moment. But if this fix is useful for your cases, it's a win already.