Pointing to the middle of an object and not its beginning can be dangerous,
because garbage collection can cause a Java object to move when it compacts
the heap. This is especially dangerous for pointers to arrays.

To avoid this, keep a pointer to the *start address* of a Java object, and
call the KEEP_POINTER_ALIVE macro on the pointer.

Also, see the following URL for more info:
  http://javaweb/~anandp/notes/ptralive/

Also, see BugId 4084384 for more info.