FROMLIST: android: binder: Fix null ptr dereference in debug msg

(from https://patchwork.kernel.org/patch/9990323/

)

Don't access next->data in kernel debug message when the
next buffer is null.

Bug: 36007193
Change-Id: Ib8240d7e9a7087a2256e88c0ae84b9df0f2d0224
Acked-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Sherry Yang <sherryy@android.com>