rust_binder: correctly handle FDA objects of length zero

In the Linux kernel, the following vulnerability has been resolved:

rust_binder: correctly handle FDA objects of length zero

Fix a bug where an empty FDA (fd array) object with 0 fds would cause an
out-of-bounds error. The previous implementation used `skip == 0` to
mean "this is a pointer fixup", but 0 is also the correct skip length
for an empty FDA. If the FDA is at the end of the buffer, then this
results in an attempt to write 8-bytes out of bounds. This is caught and
results in an EINVAL error being returned to userspace.

The pattern of using `skip == 0` as a special value originates from the
C-implementation of Binder. As part of fixing this bug, this pattern is
replaced with a Rust enum.

I considered the alternate option of not pushing a fixup when the length
is zero, but I think it's cleaner to just get rid of the zero-is-special
stuff.

The root cause of this bug was diagnosed by Gemini CLI on first try. I
used the following prompt:

> There appears to be a bug in @drivers/android/binder/thread.rs where
> the Fixups oob bug is triggered with 316 304 316 324. This implies
> that we somehow ended up with a fixup where buffer A has a pointer to
> buffer B, but the pointer is located at an index in buffer A that is
> out of bounds. Please investigate the code to find the bug. You may
> compare with @drivers/android/binder.c that implements this correctly.