tsdemux: fix truncated output segment when seeking with a stop

In disabling the stop adjustment for negative rates in
03031037 , two instructions
were inverted resulting in the stop always being adjusted by
0

Part-of: <gstreamer/gst-plugins-bad!2184>
29 jobs for !2184 with backport-mr-2182-into-1.18-tsdemux-fix-seek-stop in 32 minutes and 24 seconds (queued for 5 seconds)
merge request