baseparse: avoid bus error when no output frames
There are media with containers exposing streams but without no output frames. In this case media processing will fail, even if there are other streams that can be processed, in case of playbin2, and fire an error in bus at EOS of all streams in playbin3.
This patch demotes the error to a warning.
This MR is related with gst-plugins-base!437
Fixes: #52
Edited by Tim-Philipp Müller