parsebin: Don't expose endpad if corresponding decoder is not supported

Submitted by HoonHee Lee

Link to original bug (#778163)

Description

Created attachment 344907
Dot graph for playbin2 with missing audio decoder

Hello All.
In Playbin3, negotiation failure is observed if we can not find available audio decoder.
Actually, ACCEPT CAPS query returns 'not accepted' in gstdecodebin3-parse.c
However, In Playbin2, endpad of a chain is not exposed from decodebin when corresponding decoder is missing(e.g. unknown type).
And then, playback works well without unknown audio type.

So, I think it is more better and good to play likes playbin2.

Thanks.

Attachment 344907, "Dot graph for playbin2 with missing audio decoder":
playbin2_missing_audio_decoder.dot

Version: 1.11.1