binder: fix handling of misaligned binder object

Fixes crash found by syzbot:
kernel BUG at drivers/android/binder_alloc.c:LINE! (2)

Reported-and-tested-by:  <syzbot+55de1eb4975dec156d8f@syzkaller.appspotmail.com>
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>