Since STREAM_SEEK_TO_END is now 64 bits it could happen that some
existing code uses it in some indirect manner and it gets truncated to
32 bits somewhere (on 32 bit platforms); try to detect that with asserts.

Of course the checked value is now a valid stream position, so
perhaps the checks should eventually be removed.

Change-Id: I4248c00d758220bdc3ee94c7aa7a7bd4c86c57f0