android: binder: fixup crash introduced by moving buffer hdr

Fix crash introduced by 74310e06
(android: binder: Move buffer out of area shared with user space)
when close is called after open without mmap in between.

Reported-by: kernel test robot <fengguang.wu@intel.com>
Fixes: 74310e06 ("android: binder: Move buffer out of area shared with user space")
Signed-off-by: Sherry Yang <sherryy@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>