Backport of "rtspsrc: Do not overwrite the known duration after a seek" into 1.18
requested to merge tpm/gst-plugins-good:backport-mr-940-into-1.18-fix-duration-query-after-seek into 1.18
Backport of !940 (merged) "rtspsrc: Do not overwrite the known duration after a seek".
Commits: