staging/binder: Fix use of uninitialized variable.

Under an extremely rare condition, the variable return_error may
not be set.  This patch initializes the value to a known "bad"
value, and checks before return to ensure return_error has been
set.

Signed-off-by: Nick Kralevich <nnk@google.com>
Signed-off-by: nobodyAtall <tsogkas@ceid.upatras.gr>