Backport of "splitmuxsrc: don't queue data on unlinked pads" into 1.20
requested to merge tpm/gstreamer:backport-mr-3145-into-1.20-splitmuxsrc-fix-queuing-unlinked into 1.20
Backport of !3145 (merged) "splitmuxsrc: don't queue data on unlinked pads".
Commits:
- cddb0e95 splitmuxsrc: don't queue data on unlinked pads