SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
Remove the separate error event and merge with the done event with
error code. This then also marks the sample as done when in error
so that it gets cleaned up properly.
Fixes#486
