jni: fix java callback type

Jason Farnen <jasonfarnen@gmail.com> wrote:
> I found, since the Java write method returns void ([B)V, line 107
> should be CallStaticVoidMethod and not CallStaticObjectMethod. I guess
> some type of object was actually being returned to the C code and not
> being cleaned up, thus causing the table overflow error.

Thanks to Jason for the patch!

Signed-off-by: Joel Stanley <joel@jms.id.au>