Structure.clear() should invoke ensureAllocated()

[cowwoc]

Structure.clear() should invoke ensureAllocated() before accessing "memory". Given:

public class Foo extends Structure
{
  public volatile Pointer one;

  public Foo()
  {
    setFieldOrder(new String[]{"one"});
    clear();
  }
}

What will happen is:

1. Structure initializes memory to a AutoAllocated buffer
2. setFieldOrder() sets memory to null
3. clear() uses memory before ensuring it is allocated, throwing NullPointerException