Fix broken readNullableStrongBinder<T> implementation

Added a missing return statement.  Omitting this statement
caused the compiler to silently cast a pointer to a status_t,
which in general wasn't the expected value of OK.

Only return an error when the type casted return value is null and
the IBinder we got back was not null, indicating a type error, rather
than the expected null binder.

Bug: 30160959
Change-Id: Ief3c924b902a607335eff2fcd4d860cd8523593a
Test: Integration tests in aidl that exercise this method now pass.