End of preroll time was not updated properly.

Initial discontinuities were notified
through es_out/input_DecoderDiscontinuity
79cb45a4
using a forged empty block with discontinuity
flag.

Current decoders/demuxers discontinuity flag
is set on first block after discontinuity, and
is no longer empty data. We need to test for
this.

refs #18245
First spu dropped on preroll (preroll > INT_MIN)
(due to another special case/preroll incompatibility)