staging: binder: fix usage of uninit scalar in binder_transaction()

Fix the error path when a cookie mismatch is detected. In that case the
function jumps to the exit label without setting the uninitialized, local
variable 'return_error'. Detected by Coverity - CID 201453.

Change-Id: I6c960b7d3ad0adb28fad106a9a0b8cb934013987
Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Acked-by: Arve <arve@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>